Q: Next to message boards, where can an advertiser find all these affiliates?
A: The short answer is that you should make sure you have a presence where the affiliates are looking, or else reach them increative ways.
There are a handful of tools that will enable you to identify and contact affiliates, such as ARELIS, the Internet Success Spider, and Link Capture.
These tools will provide you with varying amounts of contact information. I would discourage trying to recruit by email, unless you are prepared to be CAN-SPAM compliant with your solicitations.
My personal favorite approach for targeted recruiting these days is to use direct mail – mostly postcards.
Back to having a presence where affiliates are going. Consider advertising in the affiliate program directories (i.e. AssociatePrograms.com and Refer-it).
If you haven’t done so already, a good way to get your progran indexed for your desired keywords in the search engines is to create an affiliate resource site (i.e. InstrumentProAffiliates.com).
Also, consider doing some co-marketing with other affiliate managers, whether it be a newsletter swap or a commission check insert.