One of the main ways a DJ can distinguish themselves from the crowd is by playing unique music. Traditionally this would include getting hold of acetate records (often known as dubplates). These acetates are created by using a special lathe machine that cuts grooves into a blank disc, giving a master template to be used for the mass production of normal vinyl records. Often these "dubplates" were handed to the most well known DJs to get early airplay, while the vinyl records were being manufactured and distributed. Even to this day they are popular dubplates are popular in less commercial genres such as Drum & Bass, Reggae and Dancehall.
Amateur producers/remixers and artists often upload their tunes and remixes to SoundCloud and Mixcloud websites. Sometimes the sound quality isn't great but it's a great way to get exposure and network with like minded people.
Radio and well known club DJs will often be sent Promos by the record companies. Sometimes you will find these being re-sold in smaller record stores (even though they're clearly marked as not for resale!), but by the time they're being resold you'll often find that the offical release has already been made or the record was a flop and never marketed widely! Though having said that, more underground genres would never become mass market hits and you can find the occasional gem this way.
Apart from the obvious places like visiting independent record shops and usual commercial outlets such as iTunes, a common approach for most DJs is to subscribe to a DJ pool. A record pool or DJ pool is a regionalized and centralized method of music distribution that allows DJs to receive promotional music to play. Here in the UK, the first such company to create a CD-based DJ pool was CD Pool (UK), and to this day they continue with CD based promos and are slowly moving toward digital downloads. Unlike some record pools, they are more aimed at provided a monthly selection of upcoming releases as opposed to exclusive remixes.
Across the pond in the USA, there is significantly more competition in the digital DJ pool arena. However, through personal experience we can strongly recommend one of the largest: Digital DJ Pool. With members across the globe, they offer great value and unrestricted downloads in mp3 (320kbps) format, and some really awesome rare remixes. They also support independent artists by providing the ability to upload mixtapes and remixes to a worldwide audience.