RF USB Transmitter
 

For some reason, I'm having a lot of trouble finding an USB based RF transmitter. I want to control some RC cars on 27mhz/49mhz via my computer. Does anyone have any suggestions? Ideally, it would also be a receiver, but not a big deal as I can use a separate receiver dongle (tons of receivers available online).

I don't completely understand what you're aiming for, but have you considered a USB-capable microcontroller and an NRF24 module for bidirectional capability?
One example: https://howtomechatronics.com/projec...c-transmitter/
This example isn't specifically USB-capable, but if you used a pro-micro or SAMD-based device, you could do something.