I use RID to treat mites, and have used it for about 6 or 7 different snakes now (my rescues always seem to come with mites!) and have had great success each time. I don't know if you have it in Canada though, I get it from Walmart.

Some things I don't keep on hand because I work for an exotics vet, and so can easily get some of it, like betadine and chlorhexadine, but it's something that one should have available. "Shed-aids" are a waste of money; all you need is good humidity or a soak after a bad shed.

I don't mean to sound snotty when I ask this, but why are you taking in rescues when you've not even had a snake before? Even though I had a snake almost my entire life, I didn't start my rescue work until after I started working at the clinic I'm at now, got more experience, and realized how badly some snakes needed to be taken away from their stupid owners! If you don't have experience caring for sick ones, you'll need to make sure you have a lot of money on hand to take them to an exotics vet who actually knows what they are doing.

Some things I have specifically for rescue work, that the average owner doesn't need:

Repta-boost for carnivores/insectivores (I either inject it into whole prey or tube-feed)
Calcium and multi-vitamin powders (used for bearded dragons/tortoises)
spare UTH and thermostat for quarantine
feeding tubes
syringes


I know I'm forgetting things, though!

Also baby oil works too, for the snake not the bedding.
You don't want to use any type of oils on a snake, as even just a few minutes of it can cause their scales to slough off. I've taken in 2 rescues that were "treated" with oil, one was baby oil and the other was mineral oil. Both snakes' scales shed continuously but each scale just flaking off by itself, and didn't resolve til the next normal shed. Oh, and both still were loaded with mites!