Also - many newspapers will sell remnant ad space to you at a discount.
Remnant ad space = ad space that hasn't been sold by them and they have to go for printing soon.
Mike Winicki has written a cool report on buying remnant ad space (the trick is to send your ad and a cheque before hand). He had also published it in the offline newsletter that was published by him and Michael Ross.
I'm not sure if Mike visits SowPub regularly anymore - if he does, hopefully he'll read this post and elaborate. Or post a link to his report.
Or else, you can contact him at his website:
http://www.bigfatnoise.com/
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You could also try out
http://www.mediabids.com