Hi,
A couple of good links for "DJ Only" type compilations are:-
DMC WORLDAnd
MASTERMIX DJThese compilations are quite expensive but they do contain some of the better tracks for D.J's. It all depends what age group you are aiming for. If you are looking to play to a Family audience (all ages) then you'll need a wide & varied selection of music. A good place to start is the 60's, 70's and 80's box sets from Mastermix, along with several of the more popular "classic cuts" Party series.
Subscribing to the DMC is also quite good if your require the latest chart / dance tracks since they have a pre release disc which comes out ahead of the commerical release so you are one of the first to get new material!.
But if you aim to do Weddings, 21sts, 40ths etc and general mobile then you'll need to build up your older collection of music and "tried and tested" party tracks first