I understand that AT/green followers do NOT work in 20 round magazines.

1. Are there any quality affordable rebuild kits for 20 rounders?

OR if not

2. Where do I get good replacement springs for 20 rounders?

check with Tommy at Georgia Precision. he has everything AR related and is great to work with. visit his site or call toll free

Yes, I missed the obvious. I still wonder about follower replacements (though I understand this is more often optional than required)

Pulpsmack:

If your 20s have the dull metallic aluminum alloy follower, you already have the best, IMO, unless they are showing white corrosion.

The green follower craze (IMO) is one that was born out of the "if some is good, more is better" mindset.  The green followers were developed to address an issue with 30 round mags, Sanchez in particular.  The black follower tended to dip in the rear, lowering the cartridges and causing sporadic FTC (failure to chamber) issues.  This was partly due to the curve of the mag and the way that the rounds stack in a mag due to the case sidewall taper.  The green follower eventually was adapted for new production 30s as well.

The long leg on the front of the green follower tends to limit rearward tilt of the follower in a 30 rd mag.

Regarding the 20s, they are straight and do not have the same issues as the 30s. Moreover, the metal alloy follower in the GI 20s is rigid, durable, and to some degree low in relative friction when in contact with brass.  There is no technical reason to insert green followers in a 20 rd mag.  

As I stated earlier on, with all the info, disinfo, and urban legend on the net, some believe erroneously that green followers should be installed in everything, and that is simply fallacy when it comes to GI 20s with alloy followers.

If you have commercial 20s with black followers and they work, don't worry about it.  If they don't work, look in EE for somebody selling alloy followers.  Mind that they are probably removing them from perfectly good 20s and installing green followers or perhaps black followers from 30s in place of the alloy followers, but that's their problem.

HTH,

Noah

Contact CMMG, they have specific green followers for 20 round mags, together with springs, floorplates etc.