You can make your own cemitas, if you can find a source for papalo or Papaloquelite. Papalo is the herb that gives a cemita its unique taste, although the chipotle sauce has a bit to say about the flavor, too. It's like precolumbian cilantro, but from a totally different plant family. I suppose you could substitute cilantro for papalo, but enough substitutions and you'll have a carnitas burger with chipotle sauce and not a real cemita autentica.
Making a cemita should not be too difficult, if you already know how to make bread and chipotle salsa - just follow the sketch above. The buns are more of a French style lean dough than a fluffy, puffy bun style dough or or bolillo like they use for tortas. I would start by adding tomato paste to canned chipotle to reduce the burn factor, then experiment a bit by adding perhaps a bit of cumin, a bit of garlic, a bit of Mexican oregano. Quesillo is a type of string cheese. The meat is relatively simple, and all that's left is some hefty slices of avocado.
You can vary the meat, too. Although I like carnitas, you can also use milanesa (pounded, breaded chicken or veal). There are so many ingredients, you could probably even leave it out for an ovo-lacto version.
If you're not going to make your own, and you're in Los Angeles, my favorite place to get this things is at Cemitas Poblanas Elvirita, 3010 East First Street, across the street from the cemetery at Evergreen. You may not see the place at first; it's kind of nondescript. It's pretty Spartan inside, too. Some tables, a big menu on the wall, a counter, a cooler along the wall and someone to take your order, preferably in Spanish. You can get the chipotle sauce on the side to let people unaccustomed to this sauce ease in gradually. Better yet, if they don't like it that leaves more for you. Beware, though. Some people's intestines don't take kindly to this kind of shock. For seasoned chiliheads, it should not pose any problems. I like the carnitas because it's moist, but the chicken - breaded like milanesa - is a good choice, too.
Alas, there don't seem to be any cemitas served in Sacramento restaurants.