Give them access to your product to show that it's really good. Show them your sales copy so they KNOW it'll convert before it even gets tested (ideally, you'll have hired a proven copywriter).

Give them 100% commissions, or at least a much higher rate than they'd normally get. Ethically bribe them if you have to.

You'll only need them to mail a segment of their list, like 1-2K, to get some good numbers to work with.