Help finding vintage parts

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I recently posted about a fixer upper I picked up on eBay. I sent the watch along with two donor movements to my usual watchmaker and unfortunately they all have a broken fourth wheel. The movement is a cal 320 from the 1930s/40s. My watchmaker has messaged to say the usual places for parts don’t have what is needed. Does anyone have any ideas of where I might be able to look for the part?