You can't buy the running coupler, you make them yourself as and when you need to.
The running coupler is, if I can explain it in words, as follows. imagine you have to join two pieces of conduit by the running coupler method, and you have a MALE end.
On the piece you wish to join there is also a MALE end, extend this thread by tonjust over the length of a coupler and a locking ring or lock nut.
Run the lock nut on to the long thread until you run out of thread, now run the coupler on, you will be left with a MALE end on the conduit that is fixed to the wall, now install the conduit with the coupler and lock nut until the MALE end and the conduit with the coupler and lock nut meet, now turn the coupler onto the MALE end and then lock it in place with the lockring.
I hope that is understandable