Have not been able to find magazines for conversion kit .22LR
Anyone has a good source?

There have been several kits on the market for several years and unfortunately not all accept the same mag.  A detailed description or photos of the conversion kit will help determine the mag required.

Like Vascar says, cannot help much if we do not know what you have.

Hopefully you will get Lucky and can use SW M&P 15-22 mags.

It is a CMMG BOLT CARRIER

Now the AR 15 its a Sig Sauer with 1/7 twist.
How good is it going to work?

Im buying it used for $140
Or should I go for an dedicated upper?

Since it is a CMMG Bolt conversion any mag designed to work with the Ciener Atchisson design mag will work.  Black Dog machine sell a couple different  generations of mags for this bolt as does CMMG.    Many users of the CMMG conversions and dedicated uppers have bought the Better Mag adapter from Redi-Mag.com and use the S&W 15-22 magsazine which gives you a fully functional mag catch.  The BDM & CMMG mags use an extra tall follower which keeps the bolt open after the last round is fired.  When you remove the magazine the bolt goes closed plus you can not lock the bolt to the rear using the bolt catch.  

I personally use BDM and gray CMMG mags and my conversions and dedicated AR-22's are very reliable.  The worst barrel twist to use with a 22 conversion is 1:7.  Accuracy varies from barrel to barrel and ammunition used.  For shooting steel plates or shilouette targets the 1:7 twist will work but for shooting precission groups the 1:7 will have a lot of fliers.

I have a couple of conversions but I found I shot my AR-22 a lot when 22 rimfire was more available so I eventually bought dedicated uppers.  The worst part about using a 22 conversion is the filth assciated with shoot 22 rimfie, plated bullets help but it is still very dirty.  You need to fire a couple of centerfires rounds for every 1000-1500 rounds of rimfire  to keep the gas system open.

I've bought BDM & CMMG mags from Brownell's, PSA, 44 mag.com plus Midway and many other vendors sell these mags.

Motoadve, if you plan on doing a lot of .22 shooting then go with the CMMG barrel or complete upper! i first bought the conversion and it shot good but just not as accurate as i thought it should be so i bought a barrel to install on an upper i already had and what a difference! i can put a full mag inside a fifty cent piece at 25yds all day, every day and twice on sundays!

and as for mags the best you will find are Black Dog Machine and they come in many capacities and styles!

below are pics of my rifle, one with the .22 & .223 barrels

So you can use a conversion kit and also swap the barrel of a .22LR and should work fine then?
How much work is to swap the barrel?

the barrel is a cmmg and its easy to do but you need a barrel wrench and i forgot but you need a different part for your convertor that locks onto the barrel.

To use a conversion with a dedicated barrel you need a collar to attach it to the barrel.

Dave N

yes, i have a regular convertor in my pic but you will need one of these collars on the end that will clamp onto the barrel 'nub'