There's a flagstone quarry in Batavia on Route 25, south of town. They have everything from giant pieces used for front steps and thinner pieces for walkways. Also in Yorkville, Ground Effects, has flagstone.
Madison Block and Stone out off Hwy 51. http://www.madisonblockandstone.com/ Be sure to bring a vehicle that can handle the weight and gloves. They'll deliver but for me it was easier to do several trips with my Ford Escape.
JCOakley, there is a large stone retailer here in Des Plaines called Schawke. Beautiful stuff, but they're expensive. I'm actually looking for a good source for low cost/free fieldstone - untreated, uncut stone. I just can't see paying big bucks for rocks!
I had a happy realization the other day . . . I was digging back by my garage and it occurred to me that the row of flagstone pieces that a previous homeowner had placed all along the wall of the garage could easily be picked up and moved to make the path I want. It serves no real purpose just being lined up along the wall. Yay! I am my own source of flagstone. But I am glad to have the other names for future reference.
Bromberek is a quarry where they sell flagstone, either by pound or ton. Bluestone, full range, is $335/ton. they are located in Joliet, Illinois 60435. They deliver but it's expensive for small amounts http://www.bromberekflagstone.com/