Best Buy. You need to find a Best Buy with a Pre-order Kiosk. On that Kiosk, search for "Star Wars Old Republic" and you should see a 2400 cartel coin offering. Print a receipt, take it to the register, and they will print out a code for you. There are no physical cards, but I have verified this works personally.
I can verify from personal experience that NOT ALL Best Buy stores have the same software/databases on their pre-order kiosks. Of the 3 Best Buys local to my area, exactly 2 of them had the cartel items. The third, and oddly enough, the most heavily trafficked of the 3 Best Buys did NOT have the cartel items on their pre-order kiosks.