If you are looking for pre-built then you can check on places such as dell.com and just customize your own. Some advise on what you might wanna get is at least 2 to 4gb ram, 250+gb hd, a decent graphics card with 512mb memory, and a dual or quad core cpu.
If you know how to, I would recommend building a pc though. It's a lot cheaper, and you can get a better pc for your buck.