Hi Andrew,
We've got a few clients that sit in the same situation as yours. I would recommend looking into Google AdWords Keywords Tool and cherry picking keywords that fit your criteria. One thing to note, the data in Google AdWords is a rough estimate based on the previous month's data. For more ideas, you may want to have a read of http://moz.com/blog/be-careful-using-adwords-for-keyword-research
Good Luck!