Tourgy, I would imagine most hardware joints would stock some sort of "Builders Grout", ('nother name - non-shrink structural grout)
.
Bunnings stock the Lanko version, in the indoor timber section with the floor levelling compounds.
To state the bleeding obvious ( you may need to when speaking to the Bunnings dude :- ) it is completely different stuff to tile grout. :)
Cheers..................Sean