That would be a special order Lee from my supplier , and with special order comes extra prices ! Would rapid flex not be an option ?might be worth giving this a try? Thick Bed FlexiRapid Adhesive | Tilemaster Adhesives - http://www.tilemasteradhesives.co.uk/adhesives/thick-bed-flexi-rapid-flexible-tile-adhesive
its pretty new so not many have used it, weve not had that much call for it tbh, we dont sell that big a tile.
see if your local place @area tiling can get some in for you?