Craft Japan, Seiwa or Kyoshee Elle are all good brands. I own quite a few of their tools. Their swivel knives are smooth. Just be careful with the shipping costs. Choose air mail if you can can or expect a month waiting time.
There is a YouTube video by a dog trainer that shows you exactly how to make one of these. He is a bit dry and his hands shake a lot, but pretty darn easy to replicate. I am sorry that I do not have the link for you at the moment. Good luck
I believe in 2 or 3 issues back in the Leather Crafters and Saddlers Journal there was a detailed tutorial on how to make this type of card slot. I want to say it was Chan Geer the author. A subscription is well worth the cost. I am sure you can always call them and buy that particular edition. Good luck
Are you able to count the number of threads? Tandy also just came out with unwaxed linen thread that you can select the thickness and wax yourself. They have a small spool and larger spool. Maybe post a picture of the spool you are using?
Not sure what you are used to but I like the straight mauls for my work. I just received a small maul from Clay Miller Tools. The biggest plus was the thick handle, which helps these old achy hands.