Any AGP based video card out there if new is going to be better than what you have now. That's good news provided as you already know can find one. Thus, it will be an upgraded version quite capable. Start at these vendors, http://www.geeks.com http://www.newegg.com http://www.computergate.com and google others.
You can use the web search engines under google, etc. by narrowing it down via the price searchers, like Newtag and Shopzilla, etc.. Of course, there is eBay, just be sure to check their reviews or credentials.
I have a five year old Medeon computer with P4 2.2ghz, 250w power supply. The fan on the video card is making all kinds of noise and rather than replace the whole computer, I'd rather find a good replacement for the video card; however, I'm not find any replacements when I Google it. Does anyone know of where to find these older generation cards and maybe even an upgrade to the one I have.