I've had good luck sourcing office furniture from:
- Landlord - If you're renting an office space talk to him, he probably has a lot of junk in storage that previous tenants left behind. Free or low cost.
- Consignment Shop - Got a killer deal here on a 3' x 5' pull out leaf table.
- Craigslist - Particleboard "office furniture" cheaper than the retail stores. Just make sure it's in good shape and doesn't smell like smoke.
- Custom Cabinet Maker - If you're doing a home office it's worth it to spend the extra bucks to make it look nice. You have to live with it after all.
I've never been to Ikea but that sounds like it might be a good option too.