I plan to build up this page over time with my own downloadable resources as well as other suppliers that I recommend. Feel free to contact me with suggestions or add them to the comments.
Here is my template to make cards for recording your yarn dyeing. I describe how to use these in this post. I print the cards out on Avery 8371 perforated business card paper, but you could also just print them out on heavy card stock and cut them apart.
Dharma Trading Company – for just about anything you could want relating to fiber and textile crafts. I use their acid and fiber reactive dyes, and I also buy a lot of undyed yarn and fabric here. Their customer service is absolutely amazing. Also check out their cool tutorials.
Paradise Fibers – a good source for undyed yarn and fiber, as well as other supplies related to the fiber arts. I bought both my spinning wheel and my rigid-heddle loom here!