The counterweight seems identical to the one found on most Japanese tables. Looks like Garrard sourced the tonearm from a Japanese supplier. You can find that counterweight on e**y or find a non working, for parts table with a similar tonearm so you can salvage the counterweight from that one.
Another solution is to put on any weight that you can find and balance the arm with digital stylus force guage.
Music is a universal language.