Most of the networks have csv or xml files for the codes of their merchants in varying layouts. Buy.at, Affiliate Window, Webgains, Paid On Results, Affilinet (rss I think) all have them.
There are also a few pretty good services out there for this iCodes - Voucher Code and Offers Database accessed via webservices or ready made customizable websites. from Leeky and http://www.promotions.co.uk/ from Doug (both forum members).
Personally I prefer manual route