Don't know what you mean that it doesn't cater to fish keepers. Do you mean they won't sell in small lots?
I do not recall the name of the place, but there was one in the rockville area that didn't have quite the selection that the stone store appears to have, but I went to this landscaping place in rockville and they had no problem at all with me going to their huge piles of rock and handpicking whatever I wanted.
As for THR, as I said before, for nice pieces that you can just drive to a store and hand select $4/lbs is very fair. I had to put in a lot of time and energy, plus I could not see them up close because I was buying them on ebay, all so I could get them for about $2/lbs.
How much you put in is really up to you. You obviously want it to be stable, you want enough places for fish to hide (assuming mbuna that means a good amount), and you want to have some room left for fish to swim. Beyond that it's all aesthetics, which is, of course, subjective.