There's some places that specialize in dj products, moreso than production stuff.
One is
http://www.pro-mix.com in Chicagoland. There are others. I say pro-mix because they sell music, although I don't know how much they have or if it's all karaoke.
You could also go the used route.
Music is the most difficult thing to acquire, because you'll need so much of it, but you may never need any of it. Say you buy an album that you think you'll
play a lot. But you never do. But to be well rounded and have a good variety, you will need it. Multiply that times about 5000 songs. You can download, but do it legally. Plus, for new stuff there's a lot of services to look into, like TopHitsUSA, Promo Only, and some others the names of which
escape me.
If you're serious about the dj biz, go to
http://www.start.prodj.com a chat board which always had good advice for me.
Also is
http://www.djgold.com,
http://www.djchat.com and
http://www.discjockeyamerica.com/ and there are others beyond that.