david weaver
Atoma 400 - cheapest place you can find it. Good for any stone in the 700-ish grit range or above (waterstones below that aren't particularly practical for tools, anyway).

Flatten as often as you'd like. I used those stones for a long time and liked them (the pros), as long as they weren't allowed to load. I'd say that even though they're spray and go, if you leave some slurry on the surface (which will help them to not load or glaze), spray them a minute or two before you use them because the slurry will harden and that's not a great thing.

Great stones. My favorite of all of the synthetics, and I'd hate to guess how many stones I've played with.

I glued mine to kingwood, which was a good idea (I got kingwood scrap for a song, not suggesting that specifically -something dense and inexpensive, whatever it may be). They are nicer to use if they're glued to something with some weight (and it has to be something stable - anything quartered and reasonably dense would be fine). Traditional bases on japanese stones are cedar (now plastic), but you'll appreciate something a little heavier with these. Contact cement, silica, epoxy - all are fine to use as the glue.