Opt-in form on your affiliate page. Find other affiliates (google search, ezinearticles etc.) then do a whois on their name. Time consuming and 90% of product owners cannot be bothered but this will get you good affiliates.
I would also say put an opt in form on your affiliate page. You can also set up a squeeze page and offer an affiliate marketing newsletter for affiliates and promote that. Then introduce them to your affiliate program. Mike