Using a search engine like Google, search for “affiliate program” + [product name], or simply email the merchant and ask. You’ve previewed the product so you know its quality (given, in this case). They have a solid refund policy that you trust they’ll honor. They provide good customer support (and you’ve tested it). You have a good story to share about your experience with the product. The offer fits your audience’s needs and won’t abuse the trust you’ve built with them.
You need an affiliate program that actually matches up with your audience. That fit of interests is the most important single factor. Without that match, it doesn’t matter how much traffic you drive. You’ll struggle to convert enough of it to actually make a difference.

Go to the Google Search Console homepage and create a free account. When that's done, you'll be directed to your dashboard, which should be empty. Click on the "Add Property" button on the top-left corner, then paste your website's URL. Click "Continue" once you've done this.
Search affiliates: With this program, you'll have freelancers or entrepreneurs pay their own money to promote your offer on search results or other online advertising platforms like Facebook Advertising. While you'll want to ensure your partner is following search and advertising guidelines, this could work in your favor if your partner has an SEO background and wants to A/B test to see which ads result in the most referrals for you — and most ROI for them.