1 vial is 10 mL with 100 units per mL for a total of 1,000 units. Divide 1,000 by the number of units per day to determine how long it could last (up to about 6 months when refrigerated and not rolled/shaken/etc.) It may deteriorate before you can use all of it, depending on the daily useage.
1 pen is 3 mL with 100 units per mL for a total of 300 units; pens are generally sold in 5 packs, though if you search you may find a pharmacy willing to sell 1 at a time. Stored properly, the pen is effective until finished. And if you contaminate it (ex shoot back into it after pulling out excess), leave it out, etc., you've only lost up to 300 mL, not a whole vial.
Canadian pharmacies will require an Rx from your US vet; they tightened that up. If you can drive up there, you may be able to buy it directly.