You're going to be out a phone this weekend then unless you have a backup.
Here are the cold hard facts.
Sprint will not unlock your iPhone for domestic use. EVER. It's their policy and nothing you do will ever win you a domestic unlock. There is no third party unlock where you can pay someone to do it for you either. It's not possible.
You CAN get an international unlock, but you have to get it from Sprint. And that means you have to meet their requirements. Those are, being a customer with an active line and no credit limits for 90 days and paying the bill on time.
If you cannot meet that requirment then Sprint will not unlock your phone for you.
Pretty sure Sprint treats Mexico as a foreign country, but not sure. In any case, it's academic anyway if you aren't a Sprint customer.
Your only option would be a Gevey SIM.
BTW, the Sprint SIM card is paired with the device. The device needs the Sprint SIM card it's paired with to work correctly, which defeats the purpose of a non-Sprint domestic SIM card.