Another option is filing cabinets and doors. If you can find some flat slab doors and some filing cabinets... that might work well.

I had to fill out office furniture at one of my startups. What better way to do so than to buy used? Check with local university surpluses. I know my first desks came from there and cost $20 each. Solid REAL wood and metal legs. They don't make them like that anymore. At least not for $20! I bought 4 for $50 or $60 and even made two end tables out of a single desk. They also had filing cabinets for real cheap too and tons of other neat stuff like keyboard trays and cheap monitors and computers. Worth a look.