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To find old sets, it depends where you live. Ebay.com and Bricklink.com are good places to look. Europe usually has more of the old sets available.
Dan Siskinds sets are some of the best customs out there. His instruction cd is excellent. Check out the sets page on Classic Castle for links to his and other people's custom sets.
Wash the dusty LEGO in warm water. That works the best.