I use a combination of different sites.

Beatport.com for their massive catalog. They pretty much have everything. But there's a drawback, with so much music releasing each week on their site, really good tracks gets lost in the mix and under promoted. And it takes a lot of dedicated time listening to everything.

DJDownload.com for their innovative listening booth player. It shows the entire wave of the track and you can skip to any point in the track just like listening to vinyl in a record shop. When there's a track I'm unsure about from Beatport's preview, I go to DJDownload to try and find it to get a better listen.

Dancerecords.com (out of business now) and Amazon.com for their prices. I've found newer releases on Amazon MP3 downloads for $.99 when their $1.99 on Beatport.

I use Juno Download. When I first started DJing about 10 years ago I used Juno to buy records online.

Their site takes a little getting used to, but you can settle into a nice browsing workflow to work your way through their catalogue.

Hope that helps.

Tom