has alot of the top players and the racquets, strings, shoes, etc they use and you can buy it right there..
Also every top player has their racquet strung at max. tension which is usually around 60-68pnds. Also most players on tour use blend, i.o. two strings in their racquet; usually consisting of a natural gut string, and a polyester based string. The gut being for exeptional feel and control, the poly being for durability and extra pop.
More interesting than the strings for the pros, are the racquets. This tends vary alot from pro to pro where as strings do not. Take for instance Roger Federer who plays with a Wilson Pro Staff, that racquet has a high flex rating, it's really head light, and has a super small head size on the racquet for added precision. Someone like Andre Agassi plays with an oversized head on his racquet which gives him more power and slightly less percision, the racquet isn't as head light as Roger's Pro Staff, and is pretty stiff for added power.
There is alot to racquets and stringing...you can read TW.com for info and specifics.