I was mainly asking because I noticed some of the custom sprites appear to have some. Case & point: Alcremie.

We actually just got up a
PKMN Code Compendium on the wiki that has all of the codes for their images like Tarashia has shown. If you want the S/A/M formes they would be in the
List of Special Pokemon. Alcremie, Furfrou, and Pasovan have a url like the one you shown because they actually do not have the x/x/x codes of their own. So, in order to have the other variations for them there is the URL (the wiki does explain about them as well).