Check out Newegg.com or Amazon.com. You can search for Desktop PC's and get a list of them. Then you can narrow your search based on price. Both websites are rated at the top 1 and 2 websites for electronics in one of the Consumer Reports magazine.
As for "refurbished". be very careful. Check the warranties carried by the seller, and make sure there is some kind of return policy. Sometimes these are great for cheap but good computers... other times... a nightmare. Additionally, make sure that the computer is wiped and either given to you (after telling you) with a blank ready to use hard drive, or freshly installed operating system.