If your spare GSM phone is unlocked then you can buy a SIM for it from T-Mobile or AT&T in the US. Check their websites for details. If the phone is locked then you'll need a phone plus SIM combo. Sometimes these carriers offer online deals but if not you can buy such combos in the US at stores like Wal-Mart, Target and 7-11. Your biggest challenge may be to get the US phone before you leave in order that you have the US phone number that you want your Fido number forwarded to.