Honestly, by me, each of the 5 LQS'es has its own theme. So if I know I'm looking for a RK batik, I call Quilter's Destination. If I'm looking for a 1930's repro, I call Pieceful Gathering. If I'm looking for a printed fabric from Japan or Denmark, I'd call Linda Z's. If I want a seasonal/holiday print, I'd contact Quilter's Heaven. If I want some fun country print, I'd call my friend Heather who runs Buttons & Bolts way up in Wisconsin so I could get it at the next guild event. And if I just want to be given a hard time, I'd call the 5th quilt shop in my area (seriously! with 4 competitors, I just don't get why an owner would be consistently rude to customers; plus, I don't know if they're ignorant or just mean, but I'd get random, wrong information from them like "linen & cotton are identical; they both come from the cotton plant"). Not cool, Quilt-Shop-That-Shall-Remain-Nameless. Not cool.
Anyways, if a LQS doesn't have it in stock & isn't ordering it, I'd just buy it from an online retailer. There's only been two times when I couldn't find a fabric -- both by Benartex. The first one was a Caryl Bryer Fallert-Gentry design so I checked her website & she said where to buy it at a bricks & mortar store in Texas -- that kindly shipped it to me in IL. The second time I just emailed Benartex & they tracked down the Holice Turnbow stamped wholecloth kit I had been searching for high & low; it finally showed up at an online retailer who sold out almost as quickly as they got some in.
The other thing I've used a few times when I didn't even know anything about the fabric was the image search site
http://tineye.com