If you can tell us what part of the country you're in and the nearest big city, someone may be able to guide you to a local source or two.
I'm in SE TX and finding decent pork bellies down here is not easy. If you don't have a local market that carries them as individuals, you may be able to get a meat manager to order them for you. But, be prepared to buy a case, which is usually 6 bellies for most packers. I've tried this at HEB, Kroger and Randalls (Safeway) and all said "No!". I now have a very cooperative local chain manager who is trying to get Smithfield bellies for me, by the case.
Since you're in HEB country you might see if there is a Central Market nearby and check with them.
You might also look for specialty meat markets and Asian markets. The price will probably vary quite a bit, along with the quality.
As far as online, I pursued that and the only sources I've been able to find that ship are SRF and Heritage Foods
. Both offer only Berkshire pork bellies, which are probably superb, but also pricey and then there's the overnight shipping charge.