Custom PCB for USB to Serial output?
As mentioned in the comments what you're looking for is a USB to TTL serial adapter. I normally prefer the FTDI based devices because they have good driver support and a direct driver option that you may find useful in the future. From the FTDI USB TTL Serial Cables page the main choice from their range is the kind of connector and the interface voltage. The Uno R3 is a 5V device so assuming your RF modules are the same maybe a TTL-232R-5V would be a good choice:



Many similar devices are available on e-bay, just be sure to check the interface voltage and the chipset used. The PL2303HX devices also seem to have fairly good driver support so they might be worth considering as well and they tend to be a little cheaper. An exception to the rule might be if you are using very low cost OOK / FSK transmitters that don't have any RF protocol handling built in such as the RF Link Transmitter - 315MHz:



These devices prefer a DC balanced input and while it is possible to get results with careful use of a UART due to that requirement and skew it's not an optimal solution. If you're using one of those styles of module I'd recommend purchasing a second Arduino and using a library such as VirtualWire that will take care of that side of things for you and also adds some error detection.