Basically it's available in asian/European countries.
You can order a TX/RX combo from any online store, except in the US.
40mhz there in the US is alloted for the Military or Police (i thinK), that's why it is illegal, but if you are using it and can get away with it, just be careful.
It's just that you are dealing with the Federal Gov't and punishment will be harsh. They could easily find out where the signal is coming from (triangulation) now, also with the help of high tech gizmos, they could spot you like a needle in a hay stack.
One racer is using it, but if a lot goes to that path and causes problems then, you better be prepared to point out your buddies using 40mhz so you will have a partner in playing chess while inside the pen. hehehehehe
I use 40mhz here in PI,but I plan to shift to 75mhz. Its getting congested
Lastly, you could use 41mhz crystals on a 40mhz TX, that way if you get caught, technically you are not in an illegal freguency!!!!! or Band!!
Peace to all and Happy Racing!!! Merry Christmas!!