Then why are they not located everywhere?

I mean Ishamael was awake and active for many decades during the 3 millennia between the sealing and the current time, and had hordes of darkfriends working for him (and the black ajah).

If SG rocks give some special power to the shadow, then why would Ishy not have arranged to have that material everywhere? I mean how hard would it have been for black ajah to arrange for there to be SG material in Tar Valon. Heck, why not in the foundation/walls of the White Tower itself(then again, Shaidar Haran did seem very powerfull there in the Mesaana encounter)?

Certainly Ishy could have had SG stone to have been used in all major cities during the time Hawking ruled. Ishy had him under compulsion afterall. That would certainly have given him the power to include SG stone in buildings and walls and so on of a lot of cities.

And even now, why just BT? With traveling commonplace and lots of darkfriend Asha'man and Aes Sedai to use, Moridin could be hauling so much SG stone everywhere that he risks running out. If SG stone gives the DO power in areas where it is located, then why is this not happening?