For those digital DJ's in North America, I highly recommend:

http://www.2thebeatdigital.com/

It's due to be launched any day now.

IMO, if you use a track to play out at a gig, you should pay for it. The exception being if the track isn't available on vinyl anymore and idiots are charging way too much for a used copy, then I say go ahead and download the track from Soulseek or someplace like that. Or if you are looking to download some mixes from shows, etc. People just need to think beyond their selfishness and look at the larger picture. Just my opinion though.