I have a Varifold folding propeller, probably quite similar to yours, and checked with the company (which is located in England), and they indicated that a replacement zinc costs 45.00 and is available at several distributors in North America.
I spoke with the guy who does maintenance on my boat, and he indicated that it's not necessary to replace the folding prop zinc, as I already regularly replace the two zincs on my prop shaft. He says that protects the prop as well as the shaft, so to replace the prop zinc as well would be redundant. Fortunately, the two zincs that I place on the prop shaft are available locally and are really cheap.
Hope that helps.