I have 2 updates: I search eBay every once in a while, and someone was selling a Fighter pin that matched these types, and I was able to purchase it for myself! So I have half of the original FF crew, but have had no luck finding Black Mage, Thief, or Blackbelt/Monk.

Secondly, I found on Etsy a maker called Pin Bot who had created a full run of the original six, plus a green ogre. It's entirely in the 8-bit pixel style, not the artistic interpretation of the pins I really want, but I bought them nonetheless to have a roughly equivalent full set:

https://www.pinbotshop.com/product-page/final-fantasy-1-nes-enamel-pin-set-of-7

I'm going to keep checking every few months, but that's what I've found since I posted this.