Hi there,
In addition to the nice modern Hall Effect current sensors out there (we didnt have them in my day working in the industry, we had to make our own or buy elaborate DC current transformers or else design AC current transformers when possible) and the commercial current shunts, you can make your own current shunt using ordinary hobby grade brass strips. If i rem right, the brass has resistivity very close to 4 times that of copper, so you can use it the same way you would use copper but expect 4 times the voltage drop per unit length.
I've made my own brass shunts and they work pretty nicely. You have to calibrate them yourself but that's usually not too hard. One relatively short brass strip makes a nice 20 amp current shunt.
If you are interested i'll look up the spec's on one of the ones i had made in the past.
Total cost, about a dollar or so.