As far as I know, Colt only made 10 round magazines for their conversion kit.
Don't expect any degree of accuracy from anybody's conversion kit for the AR15. The bore (.224) is too big for the 22lr and the diminutive 22 bullet will bounce its way out. Also, if you've got a 1-7" twist, it's too fast and will cause the 22lr bullet to deform. I've got a Colt conversion kit (for a while, you couldn't buy an AR without the conversion kit) but have never used it for that reason.
It's best to get a dedicated .22lr upper to go with that kit. Accuracy Speaks produces one (but it's about $700) as does Olympic Arms (about $500).
When my issue of the GunList comes in, I'll update this posting if I find any purveyor of magazines.