Here is a really useful map of EVDO coverage:
http://www.evdoinfo.com/images/stories/sprint2006_04USAmap.png
I've been using my Sprint phones as modems for almost six years... since the first-generation data network where you were lucky to get a 14.4 connection. 🙂
Tethering your phone with a USB cable is cheaper, simpler, and more universal than a special card (because it will work on any of your Macs, because it just requires a USB port). With any EVDO-capable phone, you will have to pay for a phone-as-modem plan ($40/mo on top of your voice service). This wasn't always true, but it's reported that all EVDO-capable phones have their data crippled until you pony up for the data plan.
You just need a USB cable that will work with the phone -- o find one, Google the model name of the phone they're lending you plus phrases like "usb tethering" or "usb modem" or "phone as modem."
You can also get the phone working as a Bluetooth modem (Google the same way, with bluetooth rather than usb). But keep in mind this will limit your connection speed, since Bluetooth's own bandwidth is much lower than the data speeds you typically can get with EVDO.
Good luck!