Track of the Wolf is a great resource for flints. But many other places also carry good English flints.
My comment on this is to by a bunch. If you only buy 1 or 2 or even a 1/2 dozen you will never get out and shoot, because you will always be concerned about your flint supply. Flints are a disposable item. As a product of nature some will last longer than others.
I always see posts that ask "how many shots do you get from a flint". Don't count your shots, throw the thing out and replace it when it's time. I remember going elk hunting with a guy a few years back. Here we are after getting up at 4:00 am, putting in over a 1000 feet of elevation gain, in a perfect spot, I look down and the guy has a piece of junk in his cock. I ask him about it, and he says he thought he could get a couple of more shots out of it. Well he didn't, and he missed a chance for a nice cow all over a $1.75. Go figure.
Sorry for the rant.
Rio