Even in St. Louis? Kansas City has at least a couple of good ones, so I always assume STL has more.
I live 20 minutes East of the Mississippi, so the StL city limits is 20 minutes away, but that's downtown in the business district. It'd be more like 40+ minutes to anywhere that might carry those sorts of stores. The only decent lumber yard is an hour away. So yes, I'm sure there are a number of art/paint supply houses that might carry it, but they're not within a driving range that would allow me to just "pop in after work".
I looked around on-line for Tints-All, and found it in Ace Hardware. We have a few of them around here, as well as JoAnn fabrics, so there are still a couple more options.
In the meantime, I was experimenting with the dyes I have, but I can't get to a good turquoise. I can get a dark teal, but I want it brighter than that.