Alright, I thought I recognized this question before. Or at least a similar one you had back in November. Your computer uses an ASUS A8M2N-LA motherboard, with an AM2 socket. Fortunately enough, AMD still uses their AM2 dies for sockets and processors. Unfortunately, everything else will be extremely outdated. Literally a decade outdated. I wouldn't even attempt to upgrade your computer. Instead I'd build a one around your budget. To answer you question regarding where you can buy processors I'd definitely recommend Newegg.
You must login or register to view this content.
With that said, I'm going to go ahead and close your thread. The answer has been thoroughly answered by Rawr, UnknownKiller, and myself. Feel free to create another post if you plan on building a computer. There's plenty of knowledge to go around to guide you through the part selection process.