I was in Yellowstone last summer, and Send It Home (the Quilt and Yarn Store in West Yellowstone) was quite nice. We stayed at the Old Faithful Lodge. I didn't really do much quilt shopping that trip; there was too much else to see! You will love it.
I agree with Feline Fanatic about being prepared for cold. We were there at the end of July/beginning of August, and it was really chilly in the evening and morning. I wished I had brought a sweater as well as my jacket.
My other piece of advice is make sure you see geysers other than Old Faithful. Old Faithful is the most predictable, but definitely not the most impressive. There are quite a few within a 5-10 min walk of the lodge that are absolutely amazing.
Have a wonderful trip!