I've been looking too, but haven't been able to source big whiteboards locally (and shipping costs on the big items limits any savings you'd get).

I went to ikea, in the "as is" section and grabbed a huge panel of lamenated something ($6). Now, i've just got to get some whiteboard paint http://www.rockler.com/product.cfm?page=17835&utm_source...

The whiteboard will end up twice as big as anything I found at office depot, and cost should be less than $40.

craigslist is usually a dearth of used office furniture. You can also often find used office furniture dealers in your area via a Google search for the same.

For whiteboards you can also use a large piece of glass (mounted on a standing frame, or against a wall).

FYI, I bought a roll of "whiteboard vinyl" a few years back at another startup with the intention of covering the walls in a conference room with it to make huge whiteboard areas. It sucked, don't waste your money. If the walls are not glass-smooth all the little bumps come through and make it a bitch to write on or erase.

The white board vinyl works best if you use it to repair some old whiteboards. You get the old ones for free, that are scratched and suffering from the can't-quite-erase-completely problem, and then cover them. You can also convert a chalkboard into a whiteboard this way.

If you start from scratch, you have to get something extremely smooth I guess -- maybe formica countertop material would work.

In answer to the general question, the best place to get used office furniture is in the dumpsters and loading docks of office buildings.