Purchased all of them at www.woolworks.com
They are called 'storage bags'.This link will take you right there.
The smallest size will hold 10 balls of most dk and worsted wt yarns. The med size will hold 20 dk and worsteds, or 10-15 of the bulkier yarns. I love these bags!
When I have decided on a pattern for a batch of yarn soon to be knit, I can slide the pattern right down in the bag, and I can see what I planned to do with the yarn!
The company is offering free shipping for orders over $50 for the time being...so if you need any yarn...or other items...now would be a good time!
I've purchased dozens of times from Woolworks. They're good people. They actually pick up their telephone, too! How's that for a novelty!?
My LYS carried them about 5 years ago. They were asking $4.50 per bag. At that time, WoolWorks was asking $1.75 for the same dern bag. Guess which way I went?
WoolWorks pricing hasn't gone up much in 5 years. $2 for the size S, and $2.50 for the size M.
They are worth their weight in gold.