The first place is local auction houses. Many auction houses sell liquidation furniture from businesses that have either upgraded their office equipment, or maybe gone out of business. These auctions often have excellent deals on furniture that's almost as good as new.
Sometimes it might even be new. After the dot-com bubble burst in the late 1990's, many of the companies that went out of business were liquidated through these auctions, and a lot of their office furniture had never even been unpacked!
Another good place to find deals on used office furniture is on eBay. You can find almost anything on eBay, and office furniture is no different. It can be a little trickier, since you'll have to deal with shipping and the fact that you can't inspect it in person if it's not being sold locally, but as long as you do your due diligence before buying you should be happy with the results.
Another website much like eBay is Craigslist. And it has the advantage of most of the stuff offered on it is being sold locally, so you'll be able to inspect the good before you buy them and you won't get stuck with a big shipping charge.
You can also keep an eye out for office furniture sales in your local classified section. Sometimes when companies are moving to a new location, or upgrading their existing offices, they will refurnish them. They often have clearance sales for the stuff they're replacing, and these generally get advertised in the local newspapers.
These sales can be particularly good places to find deals as they near the end of the sale. The selection may not be as good, but they're usually highly motivated to sell whatever's left so they don't have to move it again. You can pick up stuff for a steal at this point.
Buying second hand office furniture can be a big money saver, and it's usually every bit as good as what you would buy new. Why not let someone else pay the depreciation and buy your next desk or office chair used?