If your evaporator is freezing up your low side pressure is low. Doesn't matter what the outside temperature is (to a degree) the low side pressure regulates the temperature of the evaporator coils and if the temperature they have to be at is below freezing, it's going to freeze. Besides, you don't want cycling, that means you're wearing the compressor out.
You're in Ontario so you can't get R-134a unless you're licensed, but if you could the correct low side pressure would be 30 psi or a little higher (adjust higher as ambient temperature increases--once you have it at the point the frost is melting to dew at the evaporator outlet, you're set):
So you'll have to make do either with Duracool/RedTek "12a" (both are a blend of Propane and Butane--you could make your own blend but you're going to spend a lot of time figuring it out). This stuff is about the same as R-134a, just a little lower pressue:
Want to do it cheap and safer than the above stuff? Take all the old gas out, draw a vacuum (you should do this for the propane stuff anyways, really), and add R-152a, difluoroethane, AKA canned air. Yup, that's right, most of that duster gas you see on the shelves is this stuff (if it's not, you might be really lucky and find R-134a--it's legal for non-licensed people to buy it as long as it's intended to be sprayed directly into the air... ...dumb eh?). CAS 75-37-6. Here's the P/T chart:
25 psi is a good place to start for this stuff. You will need a side tap for the can, check eBay. The mastercool one fit well for me. You won't need much to fill the system, a lot less than you would have used with R-134a. Don't remove anything but an empty can from the tap. 25 psi might be too low for the low pressure switch to trigger, so you may need to adjust it. Have a look in the service manual to be certain. You don't want short cycling.
If you're leaking, though, before you do any of this, figure out how much oil you've likely leaked and replace the leaky part, and add oil (service manual should tell you what weight, you will want PAG oil, I recommend you make sure there's A/C dye in it).
My suggestion is you make a trip to the US. You will save a lot of money and you'll be able to buy real R-134a, just sitting there on the shelves, in convenient cans.
Items you should pick up (yeah, I know, Harbor Freight is Princess Auto quality, but you're only doing this once...). This will save you WELL over $500 vs Canuck prices depending on the parts you need to buy:
- A/C gauge set (about $50)
- A/C vacuum pump (get the one that goes on your air compressor, about $15)
- A/C disconnect tools (ONLY get the scissor style metal ones, DO NOT buy the plastic crap)
- PAG oil /w UV dye
- Orifice tube
- Accumulator (measure how much oil was in the old one and add that to the new one)
- Whatever part you discovered was busted
- A/C dye detection light (any UV light will do)
- A/C valve cores (if you need them)
- A/C valve core removal/installation tool (if you need it)
- Side can tap
The lines will probably be stuck. Since you have duster, turn it upside down, and spray only the MALE end until it's completely frozen while you undo the connection (wear gloves).
You should be able to find all sorts of guides on fixing your A/C system on the internets, so I'll let you have at whichever one makes you happy.
BTW: That RedTek setup? Yeah... It'll work in a pinch. You still need to add more refrigerant if you're freezing up, though. Either that or something's gone bad in the A/C system and it's so gunked up the evaporator isn't seeing the right pressure (which is why you need the proper gauges, the high side pressure in this case would be through the roof).