I thought all iPhones in the U.S. required a 2 year AT&T contract? So, not likely you'll be able to get one unless you're buying it in a private sale and then you have to jailbreak and unlock it.
You can order directly from Apple and use it on SaskTel, or you can buy a used Telus or Rogers phone if you're going to jailbreak/unlock anyway, or if you're not in contract with SaskTel, you can get a subsidized one from Telus with a 3 year contract and have the same coverage as you do with SaskTel. (making the assumption you don't want to go with Rogers since their SK coverage sucks)