I have called around town trying to find them. For starters the places that know what I want and sometimes have it call them side pork. They always correct me and tell me it's side pork, not pork belly. So apparently at least here in Minnesota, that's what they're calling them. You might try asking for that side pork and seeing if they have a way to order that.
Our food co-op with meat counter can get them with a week lead time, and 10lb minimum order. Also a couple of old fashioned butcher shops, but they all had 30lb minimum orders.
One place, Finer Meats in Minneapolis, had it in stock with no minimum size purchase, they do have a Mexican lunch counter on one side. So I guess you'd have to call them a Hispanic sort of shop.
Also the Asian grocery near my house has them but slices it thin as soon as it comes in. So that's not too helpful for me, and the language barrier keeps me from getting much further with them.