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Abe is generally the only place I check, besides Amazon Marketplace: http://www.abebooks.com/servlet/SearchResults?an=stein+rives...

Also, don't worry too much about getting the latest edition -- for the core things, it won't have changed much.

If you do wish to purchase new books, get them from overseas. The "let's make an expensive US version and then a cheaper but identical international version, the latter being illegal to sell in the US" practice is horrible, and it is your moral obligation to circumvent it as best you can.

I'm embarrassed to admit I didn't know about half.com. That is pretty cool.