I think only the Oohira & a few other mines are still operational. All the others are bulldozed shut, I think.
THere are a few shady high-volume eBay sellers (I won't mention the name, but I will say that if you see a Nakayama for $50 on eBay, watch out; there is no such thing as a cheap, quality razor Jnat) out there who will actually paint the skin on the back (1 way you can sometimes guess at the mine, although it really takes an expert to tell) to make it look like say, a Nakayama. Then they (I think) will lacquer over it so the paint doesn't rub off & call it a Nakayama when in fact it was scrap originally.
Something like 90% of stone quarried in the mines was rejected & left behind--these same shady types will pick over this stuff, grind it into the shape of a stone & call it Nakayama or Okudo or Oozuku or whatever & sell it cheap on eBay. Sometimes you get lucky & it works on razors, but a lot of the times, it's a paperweight.
One thing to watch is any name brand stone going for a very low price, like a full size Nakayama for $60 or an Okudo suita for the same; the latter starts @ $500 according to So Yamashita, for a quality piece. Good Nakayamas start @ probably around 300ish for a razor sized stone. You don't have to spend anywhere near 2500 for a really good finisher, but they will set you back a few hundred always, for a good stone.
There was a great post here about a year ago where someone detailed even more of what these shysters get up to & how to avoid them, but I can't find it, I'm afraid.
As far as trusted sellers, here are a few that I'm personally aware of, in no particular order:
*Alex @ the Japan Blade--member here based in Sonoma
*Maksim @ Japanese Natural Stones--also a member here based in Denmark
*So Yamashita @ Japan Tool--not a member here but many have bought from him & are very happy
*JimR--member here & runs the blog "Eastern Smooth". Recently translated "Honing Razors & Kamisori" into English.
All 4 sellers are trustworthy, sell good stones that are what they say they are & will never steer you wrong.