Your best bet is to kit bash.
I did make some legs that would be able to attach to the side of a tank hull
And a full set with a Chassis. Most tanks could just fit on top if you leave off the tracks and make a slot for it
I've also recreated most of teh Gear Krieg walker line officially in 1/56 (in their shop). though I may be able to start selling the printed kits very soon.
An M3 Lee walker/hover tank is actually something on my to do list for another client.