http://www.wowtextures.com/ - most of those should be game-ready
Any texture you use/create needs to have dimensions that are a power of 2, be in tga format, and have an alpha channel.
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"You'd be surprised how many people violate this simple principle every day of their lives and try to fit square pegs into round holes, ignoring the clear reality that Things Are As They Are." -Benjamin Hoff,
The Tao of Pooh