There are a ton of cheap 315 and 433MHz RF units out there, they operate in the "unlicensed" band which no longer requires FCC type acceptance for the hardware, but they do still require conformance testing to satisfy FCC part 15 rules (intentional radiator for Tx, unintentional radiator for Rx). I suspect few to none of them are actually tested by the makers or vendors though. If they are, they sure don't advertise it. So, if you purchase some and pay to have your own conformance testing done on them at a certified test house, they'd be legal (assuming they passed).
I checked on the popular nRF24L01 2.4GHz units, but I can't find any info on whether or not they undergo conformance testing?