I just bought some 2.5 gallon ones and couldn't really find any used. The cheapest I found at the time was at MoreBeer, but it looks like Adventures in Homebrewing has them on sale for $69 right now (don't know if that qualifies as 'cheap'): http://www.homebrewing.org/25-Gallon-Ball-Lock-Keg-W-Metal-Handles_p_2823.html.
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