You can find lots of yarn kits here in The Creativity Patch, but if you need something that's not in my shop, here are some good places to check out.
Webs Located in Massachusetts, they have an enormous stock on hand. I've been using them since the days when we'd get samples every month. They allow customers to rate the yarn and give feedback on how to use it.
Yarn Barn of Kansas They are a lot like Webs, but not quite as big. I like how they explain how to use the yarns, and give excellent advice.
The Woolery Again, a lot like Yarn Barn, and good source of weaving equipment.
Harrisville Designs If you're looking for potholder/loop weaving, this is the place.
Fiberwood Studio A beautiful site with lots of yarn to drool over.
Gist yarns The exquisite yarns are finer, and best for rigid heddle and floor loom weavers.