You can find a few, but 5" deep moulding makes a shadowbox that tends to be more shadow than box. For that kind of depth, I'd suggest an acrylic box, and then you can use literally any moulding you want.
An acrylic box would show more of the items within, and because 5" deep mouldings are costly, as is the labor to build and finish a matching extender, an acrylic box may actually cost less.
Another alternative would be to use Framerica Boxers, which consist of cap mouldings and extenders in two depths, made to stack for any depth. I've seen end tables made out of Boxers, and they look pretty good.