Ikea. Serious.
My wife and I went there looking for a regular desk for a second bedroom, but most of their desks are computer desks of all shapes and sizes. All $200 or less, too.
Hope you've got one in your area. I hate the friggin' place, but they bring cheap furniture to the ballgame and they PLAY. Get a lingonberry drink before you hit their human rat maze--makes the time go by faster.