lm317 are cheap, ask an electronics teacher if you can have a few, order them online if you can afford to wait, or buy them at radiohut if you have extra money and don't mind getting ripped off.
Probably wouldn't find one in a computer, but the place to look might be inside the PSU. PSUs are hard to find anything in, though.
I looked in the psu actually because research showed that as the most likely place to find one, unfortunately, it used a larger regulator. Darn, oh well, I had a series of unfortunate events actually where my friend, who had two scrapped xbox360s brought over their sleds, at which point I discovered that the phr803t is ONLY in the xbox360 ELITE. None of the tutorials I watched had mentioned that so I didn't think anything of it until I got to the part where my sled didn't have the right diodes. So now I'm workin on scrounging up enough cash to buy all the parts I need for several lasers, as well as some good laser safety goggles. Haha at this point I just hope I get my lasers goin by Christmas :yh: but I'm so glad I finally know what I'm doing when it comes to lasers :yh: I love this site :yh: such a great community.