IME, "lechon" can have a somewhat different and more specific meaning at Filipino markets and restaurants, at least here in Chicago.
While the word has the same general meaning, what one might see called "lechon" is more precisely "lechon kawali," basically deep fried pork belly taht is often mostly fat. A whole "lechon" suckling pig is festival food for Filipinos, certainly. However, the recipes I've seen/tasted do not involve mojo criollo, with the kicker coming instead from the liver sauce. As was said, it is sold as plain roasted pork in Chinese BBQs. Probably not going to sub fothe PR style lechon the OP liked so much.
[I have read articles tracing the common origins of the various colonial lechons to the cochinillo asado of northern Spain, which of course is almost the same as Italian porchetta. Maybe raosting a pig is just a little too basic to be attributed to one culture, but there does seem to be a direct lineage between the Mediterranean and lechon.
The cochinita pibil, of eastern Mexico, is quite similar. Not surprising given the more "creole" cooking of the Carribean coast.]
Anyway, I've mentioned previously that bakeries often have the best lechon, both in South Florida and here in Chicago, using their ovens for double duty especially on the weekends. In addition to those already mentioned, Las Villas and Latin American (both in Humboldt park) sell lechon on the weekends. I have seen lechon under the lights at La Unica during the week, but mostly on the weekend. They will very happily make it to order if you call. Indeed, the owner usually tries to get me to order for the holidays when I'm there. The Argentine butcher at El Mercado makes it too, but it never looked good enough to try.
The market on North near Mozart mentioned previously is no doubt T&C, related and attached to the late, lamented and way-too-short-lived Brazilian La Parilla. I would bet they have very good lechon.
I also once posted a reco from a Cuban guy I met travelling, which I never followed up on and now can't find. There apparently is a Cuban owned butcher shop somewhere in HP or Logan Sq. that supposedly does great to-order whole pig lechon stffed with moros (black beans/rice) and will deliver it to your door. I've spent a little time looking at old posts and e-mail, but the info is lost for now.
It might be Los Portales, further west on North from the above, but I don't know.