I am planning my 300 blackout build and was wondering which companies how many make an adjustable gas block. any help would be greatly appreciated.

I can help some, but not completely, as I just completed my latest 300 BLK with an adjustable gas block and then threw out my gas block research notes in the trash, so I'll post as much as I can remember.  

Number 1 question:  Do you want a gas block that can completely, 100% turn off the gas (for the utmost quiet when using a suppressor)?  If the answer is NO, then any adjustable gas block may do.

Number 2 question:  Does it need to have lots of adjustment or have just 2 or 3 gas positions?

I wanted lots of adjustment as I use a WIDE variety of loads (pulled USGI 108gr frangibles to 220gr subsonic and high velocity hunting ammo inbetween).  I wanted it to be able to completely turn off the gas for those times in which I wanted the utmost in quiet.  I started contacting makers and sellers.  My old PRI wouldn't turn off the gas 100% (other than that, I like it).  I contacted PRI and was told that existing ones don't turn off the gas 100%, but their new ones will...in that case you'd best order direct from the maker and specify it is to turn off the gas 100% if that's what you need.  The Seeklns won't turn off the gas 100%.  Kies didn't reply to my questions.  

I went with SLR Rifleworks.  At $80 it did what I wanted for my new build.  Lots of adjustment combined with 100% turn off.  12 click gas settings from full off to full open.  Not the least expensive; not the most expensive.  I've now spent 2 days testing and have found no problems.  Even wrapped a rag around it to test for leaks and found none.  Today I start testing the upper for accuracy.  I did open up the gas port in my new 8" bbl so as to better be able to take advantage of having an adjustable gas block.  0 clicks and the gas is off.  5 clicks open for my 220gr subsonic suppressed ammo.  7 clicks open for my frangible loads.  Today I'll get the setting for supersonic jacketed loads (I'm guessing they will use around 3-4 clicks open).  

Finally, make sure the inside diameter of the gas block matches the bbl diameter.  Most common is .750".  Hope I've helped some.  Feel free to ask if more questions.  Oh, I'm only a used and have no affiliation with any of the companies named.  SLR did ship the gas block to me very fast.

I see you are in Florida.  Anywhere near the Panhandle?

I will recommend the Syrac Ordnance and SLR Rifleworks adjustable gas blocks.

Fired 79 jacketed and 79 frangible rounds with mine yesterday (SLR gas block).  Afterwards it was virtually impossible to turn the gas adjustment screw clockwise to reduce the gas.  I removed the screw and cleaned as best I could inside the gas block.  Even removed it from the bbl.  Finally got it so I could screw it all the way in to closed position.  I will fire it more today, but only use jacketed ammo (the frangible bullets shoot dirtier than jacketed and are lower pressure; they are made from compressed lead and bakelite I've read) and see if it gets as gunked up.

Fired 152 jacketed rounds today.  Gas block not as jammed up as yesterday, but still had to use pliers to get the adjustment screw to turn at all.  Without taking it apart and cleaning more, I cannot adjust it to OFF.  Impossible to turn off in the field once you've fired a quantity of rounds through it.  At this time, I can no longer recommend the SLR adjustable gas block IF YOU NEED A GAS BLOCK THAT IS EASILY ADJUSTABLE.  If you only need one to adjust once to balance the gas in your AR and then leave the setting alone, it would be OK.  Guess it's time to contact the maker, as my experience is not like what is listed on their web site.