Where can I find good, inexpensive yarn? What is "Mill Grade 250 Yarn" or "Mill End Yarn"?

I crochet baby blankets and donate them to Newborns in Need. Yarn can be expensive. I've checked on eBay - some of those prices with shipping are outrageous! The best I've found so far is "I Love This Yarn" at Hobby Lobby; it's $2.79 for 7 ounces, 355 yards.