For the more fitted type slipcovers: Target and Bed, Bath & Beyond carry slipcovers. I have seen slipcovers at Wal-Mart, but I can't remember if there were any for chairs. Domestications (mail order) also has some nice looking slipcovers reasonably priced. You can do a little comparison online without having to waste the gasoline.
Some of the discount stores carry slipcovers; some are the drape-type (don't really stay put that well).
It may be more difficult to locate a good slipcover second hand, but it can't hurt to scour thrift stores, freecycle or e-bay. You may find a throw or a sheet that you can use for a while.