Hi Karen,
As far as the bleach goes I would not put a drop of that shtuff in any fountain since dogs, cats and especially wildlife will generally bathe and drink from it.
As far as finding the perfect stone, go to a stoneyard (look them up in the yellow pages)and find the stone you wanted and for a fee they will drill a hole for you.
As far as algea is concerned just put a scrubbrush to it a couple time during the summer.
Plus there are places,and I think you could find them on the internet, that sell millstones. Or.....purchase your own large stone and chisel your own pattern in it. A fountain is a work of art...your work of art!
Good luck
Matteo

A small amount of bleach is no more harmful than using city water that has been treated. Don't put a gallon of it in there and you should be fine.

if you have featherstone available, it is easily drilled and you could plumb it to have water overflow out of the top as an ornamental fountain.