You can buy any standard mil spec stock and have it professionally pinned at a fixed spot. ADCO as well as many others perform this service which is nice because they ensure its compliant for your state and it gives you a little extra protection should someone ever question the work.
Any stock that requires modifying the buffer tube cannot be used on the 15-22, which really sucks. I've got a spare Magpul UBR stock sitting here that would be nice to try on my plinker. but it's not going to happen.
Good luck and let me know if you have any questions.