Anyone knows where I can get a replacement transmitter antenna?
You can use any generic 2.4GHz WIFI antenna. This is the same antenna which is used in home WIFI computer equipment and on wireless routers, etc. In fact, if your home wireless modem has these type of antenna sticking out the back of them, you can simply unscrew it and install directly on your M11x.
Alternatively, you can spend heaps on a Futaba 4PKS antenna, if you get the actual Futaba part or many brands have replacents that are cheaper.
IMHO though, one really can't go past the Team C 'shorty' antennas. They look absolutely awesome, are perhaps 1/2 the length of the original antenna and so far, in my use, even with the antenna constantly being left in the folded down position, I've encountered zero range or distance problems. If you want to know what it looks like I could post a pic of my M11x with this shorty installed. Been using for about 2 years now with no problems whatsoever.
Hope that helps mate.