I know of the Giselle Kinkylove items, but I'm curious what else there is? I'd love to find more options for leather dog masks, if any others exist aside from what my HA is currently wearing.

L'amour parlor 2020, the same place that dog mask is available is your best bet.

Other than that, you'd be looking at making your own "mask"

Take a mask, eg: Add a dog nose, eg: Add some ears, eg:

That's pretty much it for "official items." There may be some CWs, but I'm not familiar with them in general, and nothing from my limited knowledge comes to mind.

There are a few good lines to draw from for kink avies, i.e.: Police & Fashion Police sets, Giselle Kinkylove set, Raucous + Rowdy items (mentioned above), Chipper set, Incubus & Succubus sets.
To pull it all together, you're going to need to search 'leather', 'chain', etc., in the wardrobe search bar and borrow from multiple sets as well as include a few CWs.
I am not aware of other pup-play/leather dog masks on this site aside from the one you're already wearing; it would probably have to be commissioned.
Outfit Not Found
I was messing around with this the other day, lol, it's just a placeholder WIP.