I`d look at suppliers of expedition kit to start with to see if they do tie down points for a D1. Ebay might come up with something but I suspect they might be fairly infrequently for sale on there.
Maybe also try one of the big LR breakers like Equicar ?
Depending on how heavy your toolbox is and how sound your floor I`d also think about bracing across the tie down points - even if you then discount it as not necessary but best to check.