SIM cards: several large SIM networks including AT&T and T-Mobile. You can find coverage maps online, but I've found AT&T in more rural places.

Many different resellers on SIM cards that will use these networks but charge less and also support unlocked phones including several already mentioned above.

One I use is "net10" and the SIM cards are sold in Walmart stores. The package has SIM cards in different sizes and typically has *both* an AT&T and T-Mobile card where you pick *one*. Activate online or via phone. These have a month by month prepaid service. The AT&T flavor has 2GB limit over this time and T-Mobile no limit.

A net10 card bought from Walmart is what I plan after arrival back in the USA after a year and half on the road.

As far as flags go, I don't foresee problems.