If this were my snake, she'd be tubed for a good look inside her mouth. Avian speculums are probably the best tool for snake-mouth-peeking, but if you don't have one try two (CLEAN) sexing probes or one smooth metal probe tilted at an angle. Do not insert anything inside a snake's mouth that is not smooth and clean, as their oral tissues are extremely fragile and sensitive.
There could be an impaction, injury or infection inside the mouth (I see substrate so this is a possibility) or a respiratory problem. If you see no visible abnormalities in the mouth, then the latter is more of an possibility than the former. The respiratory problem could be bacterial, fungal, parasitic or even viral.
Even if you cannot find a vet who will see this animal, you may find one that will take a C&S test swab and/or fecal sample if you take the sample and don't bring the snake to the office. Ceftazidime (Fortaz) is pretty much the broad spectrum antibiotic of choice for venomous snakes, as it has a long dosing interval and can be given intramuscularly without irritation or nephrotoxicity.