ipsedixit the lay definition of matambre for cerdo and vacuno is "the muscle under the skin and over the ribs" (although it extends further back than that on the cow). At least from my experience in Brazil with cattle, it comes from how they traditionally field killed the animals, skin it, and its one of the first cuts taken off which is still practiced. Pigs were often killed at the same time, in part for sausage making with a mix of pork/beef. In fact in Brazil many times if you get beef ribs from a butcher, the matambre (or part of it) is still attached (not the same cut as our flank steak). And you can buy part of the belly, ribs, even part of the loin of Pork together as one large cut which doesn't exactly mirror our primal cut. Here pork is generally killed, cut up into primals and then skinned so not much going back from there. The flank muscle of the pig itself, I think part of it goes with the shoulder, belly/ribs, etc. And that could make sense with what I have seen for matambre de cerdo (de porco in Brazil), but not certain that its the flank muscle itself. Unless you go to a custom kill facility, not certain you can get the same exact cut. Your best retail options for a substitute might be to work from either bone-in pork bellies from an asian market (you might get more of the forward part of the muscle here) or trim a skinned pork belly well of fat (less waste for you). In any case the closer you get to the primal or whole sub-primal, the more meat you have to work with. On the beef side there are still butchers who work with skinned sides and quarters, particularly for veal, and additionally some asian markets which work from whole pigs.
stupid question maybe, but have you tried just asking a local butcher? forgive me for my ignorance, but isn't this cut just flank meat cut off the bone?
in recent years, my mother has become something of a health nut when it comes to meat. she shops at chain grocery stores only but started asking the butcher at her local gelsons to slice her meat on the spot so she could get only certain cuts that aren't usually done (or mixes of meat, or mixes for ground meat, etc.). he's happily obliged over the years with all sorts of strange requests from my mother. sometimes they cost an arm and a leg (her favorite beef cuts cost upwards of $45 per serving) but the meat's always beautiful and delicious. this is in LA too btw. might be worth just asking your favorite butcher or meat supplier... don't want to miss out on that pork!