You can try specialised websites and organisations like the BMIC (bmic.co.uk, I think, and criticalnotice.com serves the same purpose), and you can get scores pretty cheaply there (you can try your country's music information center, or the country of the composer you're interested in). Of course, this doesn't account for *all* the contemporary composers in the world, but there's a lot of music there.
About modern (i.e. more early 20th century music), generally, whenever the copyright of the composer has not expired (i.e. the composer/last author of the pieces hasn't been dead for at least 50 years - 75 in europe and a few other countries), they tend to be quite expensive, whereas scores of composers who are public domain (such as Mozart, Beethoven, Debussy, Scriabin etc) tend to be quite cheap (and published by many more publishing houses, as nikolas pointed out).
Expensive prices on contemporary scores suck. They should be much cheaper than the other scores, so that more people could be exposed to modern and contemporary music.
You may want to check sheetmusicplus.com , it was quite a few modern/contemporary scores and usually at better prices than normal retail stores (although some scores are bound to be high - Birtwistle's "Silbury Air" is at 154 dollars

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