This forum is of course UK based, and with members from all around the world someone might know of something; but I'm thinking there might be more local knowledge on a US based site. I don't know if there are any exact equivalent to Helen Howes, but I guess Cindy Peters comes close. I think she has a facebook page with her business and she handles the
TreadleOn.net web site. There's the Featherweight shop, they have parts and accessories for other models too. I think Sewclassic only have brand new replacement parts.
The buttonholer in the clamshell box is luckily common enough, and ebay is still not a bad place to look for them. As far as I know, they didn't come with the additonal templates and they were bought separately. A complete set should come with five templates (four placed in the box and one in the buttonholer) and four extra should be in a small cardboard box. I'm not sure if all variants came with the cover plate for the feed dogs, since most of the slant shanks can raise the throat plate above the feed dogs. I think the 301 needed a separate cover so I guess some at least came with one.