Where do I find 7mm brass threaded (on the outside) pipe?

Consider using a thread-cutting tap to re-cut the holes, like to 1/4" NPT, to be able to use readily available brass pipe, or "nipples" which come in a number of lengths. If that's too close to the size of the existing hole - so even a newly tapped thread would be thin and weak - consider the next size up, 3/8" NPT, first redrilling the holes per the tap manufacturer's specs (Irwin's products have this right on the package), then tap in nice clean threads.