Here are the YMM search options we have integrated with the Turn 14 app.
We have worked with Capacity YMM search ($29.99 a month) and Partfinder ($49.95 a month) mostly as they are relatively straight forward.
Convermax is a excellent option as well if you need custom search fields or additional sidebar filtering. They create custom searches. That's not to say that you can't try another app if you prefer!
Does the YMM come preloaded with all vehicle info?
We provide the fitment file in a download link for both the Capacity YMM search app and the PartFinder app.
Convermax gathers fitment data themselves, and doesn't need any data from our app. They have a bit of a set up process and will go through this with you.