Two most effective channels:
1) Craigslist.com. It's free and works.
2) Posting on MLS with a flat fee ($199 in my area).
If you still don't think it's enough, try posting it on apartments.com. But I think 1) and 2) should be enough. If agents contact you because they see your listing on MLS, you need to pay about 40% first month rent to them as commission, that's fair.