You can buy subsonic .22 LR ammo, but it is MUCH louder than an air rifle, even a loud PCP. There are .22 rounds that have only primer, and shoot with energy levels like a PCP (Colibri, I believe). I have not shot these as they will typically not cycle a semi auto rifle, and I believe the energy levels are something like a PCP air rifle.
I have owned three .22 rifles, and shot about 10 more. I had a Ruger 10/22 that spent years under the bed in a case with very little shooting. I lost it to a burglar about six years back, and recently decided to replace it. Looked at CZ, Marlin, and Ruger, read a lot, and ended up with a Ruger 10/22 DSP "deluxe sporter". Paid $270 new at a Plano gunshop, and that was a good price then, they seem to be $100 higher at the auction sites. I think you could get a base model for about $200 new if you look.
Ruger 10/22 is the most popular rimfire rifle on the market. It can be customized in a huge variety of ways. I think you can buy them at some Walmarts.
If you live in an area with little restriction on guns, you could plink or shoot the occasional varmint, but in a suburban setting, I'd say forget using a normal .22, it is much too loud. I had some thoughts that I could plink in my backyard greenbelt (where I shoot air rifles frequently) with subsonic .22 rounds, but I am quite sure that if I did so with my Ruger, I would have police visitors. I have my new rifle setup with a nice mount and scope, and changed to a Volquartsen hammer. With the extra magazines, scope, etc. I have probably $400 in this little gun. It holds a pretty good pattern out to 50 yds plus. My brother's customized 10/22 is shooting about half inch patterns at that distance with Wolf match target rounds.
See this thread:
http://www.gatewaytoairguns.com/airguns/forums/thread-view.asp?tid=14843&posts=16&highlight=ruger&highlightmode=1#M120417All this said, I think the .22 is a fun gun to plink and shoot targets with. Ammo is pretty cheap (although increasing in price dramatically). I am pleased with my purchase, but will shoot it only at ranges or out in the country.