Tapfiliate is an advanced application on its own, but perhaps you have an affiliate marketing strategy that requires a more specialized, technical implementation.
Ask your social circle - Perhaps someone in your network has a reliable reference they can point you to.
Search for a developer online - There are hundreds of freelance developers out there, as well as many listings. However, be mindful that not all of them are top-notch.
Websites like Upwork and Fiverr list developers and their rates, but their talent isn't assessed, and quality of their work is neither assured nor guaranteed.
As a general rule of thumb, avoid developers with a rate that's "too good to be true", because it probably is. You may end up with sloppy code or not get the intended result.