Hi there! If the stone is expensive and important to you, you should
make the basket yourself to the proper dimensions. Otherwise, you
can buy 12 x 10 head, and cut it through the middle until the
dimensions meet 7.5. However, I'd caution you that the 12.5 might
not fit in the 12 head either, so you may even have to go to a
bigger basket and cut that down.
To cut down, I usually saw in half in one direction and then solder
(or laser if you're lucky enough to have access) back together at
appropriate size. Then, cut in half the other direction and again
cut down to proper size and solder back together.
Don't even bother with the 11 x 9 head - the prongs won't stretch to
accomodate a 12.5 stone and you'll just wind up frustrated and
wasting money. If the stone is oval, you MAY be able to buy a round
and stretch/compress it to an approximate shape.
Hope this helps and good luck, Holly Hawkins