https://www.frenchpaper.com/ Excellent selection of paper, card stock and envelopes in a variety of sizes. Digital papers are also available for print-at-home designs.
https://hollanders.com/ Decorative papers and bookbinding supplies. Tools, tapes, linen thread, leather, blank and diary text blocks and more can be found here. Workshops are given too, when permissable.
https://www.talasonline.com/ A virtual oasis of bookbinding, printmaking, conservation and other fine art supplies and tools! If you can’t find it here, you may not need it. High quality products. Shipping is fast and the service is excellent.
One of the best collections of how-to videos for bookbinding available on YouTube. Check it out! (And don’t worry when you don’t hear sound.) https://www.youtube.com/user/BookbindersChronicle/videos