The easiest option to go about this is certainly by searching youtube. Simply think of your favorite pop tune, find out who produced the tune, and kind because manufacturers name in youtube followed by "type beat", "kind instrumental", or "kind track". This will obviously bring up a a list of producers who make tracks to the likes of that producer. Here, you are able to utilize your own judgment to see who's worth using and who needs some work. These manufacturers are generally passionate about music manufacturing since they are placing their music on youtube as opposed to those who simply have a webpage with a checkout cart next to the track. I'm not suggesting those producers are phony. I'm just claiming they are more likely to be the ones trying to capitalize on this growing demand for pop beats for sale.
Back to the topic, check those videos out and see who's worthwhile. Take see as to how many views and loves they have on their videos. This will give you an idea as to how good they are from the perspective of the common person. Once you find a solid prospect, there are always a few approaches to go about getting their attention in order to work with them. More high profile producers can be harder to reach, but these simple tactics must be sufficient to get any manufacturers attention, whether they acknowledge you or not.
I'll begin with the best approach. I would encourage making a youtube video in response to the video of theirs you love, where you have authored a track to their track. Should you decide are actually a good writer/artist, you could definitely stir up some attention. Simply download their track and make a track on it, record it, and upload a video of it, then post it as a response to theirs. That should open up the line of communication. Another less intimate approach of getting into contact with them naturally is by messaging them on youtube, or any personal network they might share with their readers. Whenever/however you get their attention, ensure to ask them how much they might charge to manufacture you a track. Any music producer will tell you that revenue talks. They;; be sure to hear to you and there you go, you have found pop beats for sale.