If you've tried McMaster and maybe MSC, I'd have one made.
I've had custom shaped end mills made and they're not all that expensive.... Yes Paris, relative is just that, relative to the cost of other tooling.
Tell your tool maker you want to start with a standard double ended end mill and have him grind yours on both ends of the mill.......... it's cheaper grinding the same profile on both ends than having two separate end mills made.
If you design your cutter with a pilot guide like a counter-bore tool, you may even be able to get away using it on a nice deburring machine.... deburring machine is what I call Bridgeport mills.