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There are a lot of places out there -- a web search will probably find more than you could ever explore. Two that I am aware of and have bookmarked are:
http://www.cadblocksfree.com/index.php
which [despite the name] has some Block collections for sale, but does also have a lot of free things, and:
http://www.cadforum.cz/catalog_en/
Also try specific manufacturers' websites.
Kent Cooper, AIA 
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