Really good question. Personally I think you need to start looking for partners while your product is still in concept. You will find that those experienced in affiliate marketing can give you some awesome tips about one time offers, product components, etc. that will make your product more appealing to affiliate marketers when it comes time for launch.

As far as finding affiliate marketers who may be a good fit, I recommend reaching out to others in your niche via blogs. You can find some really great resources and individuals who may be great affiliates.