it depends on how much music theory you know...
i do know enough not to try myself and ask for help instead (if i want something relatively well done)

My advice would be: Ask for your local music practicing locals, buy a pair of autoproduced demo cds(they tend to be cheap) and talk some of the groups whose style match what youre looking for into colborating in your project. It is probable some of them would be very interested to compose something for you if you credit them (and pay them a bit if you make money from it) and some of those groups are ****ing good indeed...