There are a ton of fantastic pen blank resources available. Here are some you may find as additional info to augment what you've found here already.
https://www.alumilite.com/ High quality resin used to cast pen blanks
https://www.exoticblanks.com/ Great blanks, pen kits, and people.
https://www.mckenziepenworks.com/ Tim's one of the best, he makes fantastic blanks with diamond powder in them. A+ guy, too.
https://www.pennstateind.com/ Good source for pen making supplies, though they are a little slower on shipping (and a bit more pricey than alternatives), but they have a ton.
https://www.woodcraft.com/ Retail store with lots to offer. My local guys are helpful, friendly, knowledgable, and they know me quite well by now! haha.
https://www.rockler.com/ Similar to Woodcraft, retail store
https://www.woodturningz.com Cool site, cool stuff. Lots to see
https://www.arizonasilhouette.com/ Huge selection and some really great pen kits, pen blanks, and other items that you will have a hard time finding elsewhere
https://www.penturners.org/ Great forum full of folks and lots of historic info. If you have a question about turning pens, you'll be able to get help here.
https://www.woodturnerscatalog.com/ Great resource for all sorts of material, from kits to blanks and tools.
https://mywood.shop I had to pop my own finished pen site in her. I also put blanks and other items up on this shop. You can often find finished pens made with blanks right from this site (I usually make 6 or 7 and turn 1 to sell as my own here)