I know there are PASGT, ACH/MICH and LWH.

what are the differences? what are the benifits? home defense use?

i seen PASGT's go for 189, but MICH's go as high as 900. whats the difference?

If i were looking to get one, where would i go? what do i look for?

PASGT is the name for the desert storm era helmet issued to the Army.
The standard suspension system on these is bad. They tend to come down too much  in the front (blocking your vision).

LWH is the helmet issued by the Marine Corp. It's basically a PASGT with some minor upgrades.

ACH/MICH is the name of the new helmet, replacing the PASGT. It's lighter, doesn't impede your vision as much and has a very nice and modular suspension system.

There's no contest between the 3 listed here. It's ACH every day all day. The real question is whether you want the standard mid cuts or high cuts, or some of the other spec op style helmets (now we're talking $$$)

Per Wiki....

PASGT
Personnel Armor System for Ground Troops (PASGT; pronounced pass-GET), is a combat helmet and ballistic vest that was used by the United States military from the mid-1980s until the mid-2000s (decade), when the helmet and vest were succeeded by the Lightweight Helmet, Modular Integrated Communications Helmet, and Interceptor body armor.

LWH
The Lightweight Helmet (LWH) is the U.S. Marine Corps replacement for the PASGT combat helmet.

As it is nearly identical to untrained eyes in shape to the PASGT, and outside of the Marine Corps it is still called the Fritz helmet or K-pot (due to its resemblance to the German Stahlhelm). Though heavier than the Army's Advanced Combat Helmet, its larger size also offers more protection and is lighter than the PASGT. Featuring a V-neck strap and improved fit, it is much more comfortable than the PASGT.[1]

The LWH entered service in late 2003 and completely replaced the PASGT by 2009.

MICH/ACH
The Modular Integrated Communications Helmet (MICH) is a combat helmet and one of several used by the U.S. military. The MICH helmet is the primary combat helmet for use by soldiers of the United States Army. It was developed by the United States Army Soldier Systems Center to be the next generation of protective combat helmets for use by the U.S. Army.[1] The Advanced Combat Helmet is derived from the MICH.

You can pick up a used ACH for about $200.

$900 for a MICH??   You mean $150/200

For $900 you can have a new opscore

Love my MSA ACH. I definitely have to recommend the LWH or MICH/ACH. I don't know where you got these prices from but I got mine almost brand new off the EE for $100 with Team wendy nape pad/retention system, team wendy zap pads, multicam cover and the nvg shroud. Fully loaded for $100

PASGTs are the older USGI helmet. They're usually $50-75. They're widely available, but heavier and less comfortable. although they can be upgraded with various pads and chin straps.

ACH/MICH are both the same helmet. MICH refers to a helmet system which includes the ACH as one component. Lighter and has a higher ballistic rating than the PASGT
They're available in 3 styles/cuts:
2000: standard cut helmet used by big army
2001: High-cut helmet with the ears removed for comm systems
2002: Mid-cut. With the ears cut back slightly.
Brand new, all these helmets run ~$500. 2000 cuts are widely available second hand for ~$200.
They're all also made by a host of different companies. Gentex, Revision/MSA, BAE/SDS and some other smaller contractors make these for the DOD. Rabintex also builds a version for the IDF.

LWH: USMC issue replacement for the PASGT. Same cut as the PASGT but uses more modern materials like the ACH to make it lighter and more protective like the ACH. These can be found used but it's much harder than the ACH

ECH: An enhanced version of the ACH with more modern materials that allow it to have better ballistic protection..

Now the really nice stuff:

Ops-Core FAST: Very popular modern choice of a helmet. Typically high-cut with ARC rails and a 3-hole mount pattern. Lighter with a better suspension system than a standard ACH's pad system. These are popular in the SOF (and well-funded SWAT) world. Run $900+ They can also be had in high and mid cut as well as a "maritime" verison ($1100+)

Crye Airframe: Lightweight, uses a pad system instead of a suspension system. Comes with ARC rails. The biggest innovation they bring are that they're a 2-piece helmet to allow air to flow to the top of the helmet. Run $850+

Team Wendy Exfil: The newest "cool-guy" helmet. High-cut and apparently very light with Team Wendy rails. Runs $950

There are also some "off brand" helmets. Some companies have built helmets using the ACH material to make PASGT and ACH style helmets. Many of these retain the PASGT hardware locations and come with either a modernized suspension system or pads. Pro-Tec was making these types of helmets for a while. There's quite a few to list them all. A popular brand of these are Kejo, because their High-Cuts sell for ~$300.

There are also knock-off versions of the FAST. Basically other companies use the FAST cut (slightly different than a 2001 ACH) and use more available materials and pads, and add ARC rails, OpsCore shrouds, and the same cuts of velcro to make them look like Ops Cores without having the lighter materials and suspension system. These are much closer in price to the ACH helmets than the Ops-Core. However, the companies doing this are usually foreign and much less reputable than the above brands.

whats the best place to find these?

http://tnvc.com/shop/category/helmets/

Which ones?

PASGTs and ACHs can be found cheaply in the EE. You can also find the Occasional Ops-Core and Air Frame there.

Kejo Helmets can be found at Body Armor Company

New ACHs, Ops-Core, and Team Wendy at TNVC and Own the Night

Crye Airframes at TNVC.

I've seen ECH and LWH helmets used but no idea where to get them new.

EE, eBay, Craigslist, arms list.

Based on the fact that you don't know much about ballistic helmets Ill assume You aren't military or law enforcement and that you just desire to own a ballistic helmet for self defense. A good starter helmet would be an ACH, you can find them used or around $100-200 sometimes with some decent accessories, covers etc. if you grind that you use it all the time then you can upgrade to a better helmet but don't spend $1000 on a helmet if you're not going to use it. I personally started with and regular ACH and later upgraded to a high cut so I could use the folding Peltor headphones and also upgraded the pads and chin strap for more comfort.

I got a nice msa ach from a surplus store for $90.

This...


Rabintex is GTG..... can be found at TNVC