I have bought and tested this USB transmitter:http://www.keene.co.uk/electronics/multi.php?mycode=usbfmt
from Keene Electronics, and it works fine.
Of course, the frequency is normally adjustable by driver and it only comes with Windows drivers.
I haven't yet tested whether the frequency is stored inside the device, once set from the driver.
The default frequency is 90 MHz and the range is quite limited.
My small radio already needs the antenna to be extended, at around 10 m distance from the transmitter.
I may post more details later on when I have done some more tests.