DPMS "makes" a conversion upper as well. Although I am pretty sure it is the same one that Old School was talking about. Also it really depends on how much you want to spend. I have an AR 22lr upper from Fulton Armory, that is frickin amazing! The acutron upper in 22lr. There is also a company called Kuehl Precision Firearms They make some fantastic uppers in 22lr. $$$$$$$$$$
Without telling you to look into something else entirely, as others have suggested, I have to say that my luck with .22lr uppers has not been so good. I tried the Bushmaster .22LR conversion. Coming from one of the most reputable AR manufacturers, and at the price of around $400, you'd expect the thing to work. This just wasn't the case. The thing would fire and eject just fine, but the thing couldn't feed to save it's life. I don't have any experience with the others that are out there, but the Bushy should have, but didn't, work. :?