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In the Steam Market: http://steamcommunity.com/market/listings/440/Mann%20Co.%20Supply%20Crate%20Key
or on these forums where you can get them for 1.75-1.90 each:
http://tf2-trader.com/index.php?board=7.0
http://forums.sourceop.com/forums/32-TF2-Item-Trading-Buying-Selling
I have several copies of HammerWatch for 1key or $2.00 PayPal.
Add me on Steam to discuss!