AbeBooks is a great place to buy the non-US versions of textbooks (which are almost always identical to the US version, but in grayscale rather than in color).

For instance, on Amazon, the K&R C Programming book goes for $28.52 used, $61.74 new, or $28.70 for a one semester rental [0]. For a book that hasn't changed since 1988, these prices are absurd.

While as on AbeBooks, the international edition goes for $10-11 [1].

[0]: https://www.amazon.com/Programming-Language-2nd-Brian-Kernig...

[1]: https://www.abebooks.com/servlet/SearchResults?bi=0&bx=off&c...