Mason, they are vintage, so you won't find them out in yarn shops.
Steph, you have to be specific. The majority of them come from the UK. I use these key words: (bex, bakelite) (wool, yarn, string) holder
And then sometimes "yarn holder" or "wool holder" or "string holder" You really have to dig around to find them, but it's worth it! It was my goal to have 2 of each design, well, not I've decided that I want 3 or each design (well, the designs I like .... which are most of them!). Also, they are small, each of these will hold 50 gr of sock yarn, they don't hold much yarn. Still, I knit socks all of the time and they are BEAUTIFUL to look at and a wonderful hobby in collecting vintage knitting accessories! There are these, too. and bakelite needles and boxes, all sorts of fun things!