I used to have a CF driver on my area that made a lot of money building boxes and selling them to other drivers. I don't know if EBay would have anything like this, but I would look there. I didn't think these things were that expensive anyway.
They aren't cheap, you can get an aluminum box at some cb shops for about $100 or so, a decent am/fm cd player is at least a $100+ depending on if it has sattelite or remote control even more, a cb another $125, then depending on whether the box comes with speakers, ($50.), you have to get power cords and a few other odds and ends, labor to assemble, your looking at $500.00 and up depending on how fancy you want to get.
I"ve put a couple together, it takes about 3 hrs or so to assemble one. Depends on what kind of box you use. Some guys make them out of plastic tool boxes. Some use wood, more time involved in that though.
Occasionally I have seen them on ebay.
I've also seen some people just mount a cb and a radio on a board and hook up the wires to the tractor. Most tractors have power and speaker connections, with the exception of the old internationals.