Maybe most of you know about Colourmart but just in case you don't I wanted to pass this on. (https://colourmart.com/). They are in England and sell high quality mill ends are low cost. The prices include shipping and you can get free samples if you put an item "into your basket" and ask for a sample. Most of their enormous inventory is on the finer side, e.g. not rug weaving oriented. If you get on their mailing list they will send you notices every few days as to what's new. You can put items in your basket and leave them there for days before you need to commit or change your mind. Their samples are huge: enough to actually dye up and weave.
Their website takes a bit to learn how to navigate but there are loads of natural fibers: cashmere, alpaca, silk, linen, rayon, cotton, merino, etc.