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Go to Amazon and buy the sim cards now the when you are ready activate one of them. This is what I did and here is what I bought. You can activate the service online with the sim and the call the porting department to port your number over the temporary number.
(2 Pack) Authentic AT&T ATT SIM Card Micro/Nano/Standard GSM 4G/3G/2G LTE Prepaid/Postpaid Starter Kit Unactivated Talk Text Data & Hotspot (+ Free Tray Removal Remover Eject Pin Key Tool)
Another Amazon 2-Pack option for future spelunkers is this one (which I'm using right now): [link]
Both of those items go in and out of being available with Prime shipping, sadly, so the more known-good options the merrier, right? :)