Large chunks of black lava rock

Hi guys, i am using lava rock in my aquarium but had to use smallish chunks. Does anyone know where i can get big chunks from? Looking to do something really dramatic in a friends tank. I can't find any in the uk big enough for what i want. Its great stuff because it can be carved and shaped and its lightweight.
Questions
Does anyone know any supliers to contact?
Costs for shipping etc?
Rough sizes of chunks?

Looking into it for now but just want an idea.

Thanks!!



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