Welcome to Geekhack!
Another happy FC660C user
As Ludovician said, replacement keycaps for Topre keyboards are few and far between, and unfortunately Leopold chose a non-standard layout (if there is such a thing as a standard Topre layout, with the very few Topre keyboards available), making it even more difficult to get replacement keycap sets.
Many artisans produce Topre variants of their work, and one or two guys are working on making "ordinary" replacement Topre keycaps in various styles and colours, o there is still hope!