I assume this series if Thunderbass has black tolex abd isn't the earlier two tone beige/woodgrain model.
It will be difficult to find the origional head aluminum stand-off hardware. You could made one or rig up an equivalent but you may not be pleased with the results.
I've had both the two tone and black tolex models and know why may people who have these amps remove the OEM mounting hardware.
First off...the mounting bracket on the Thunderbass head has a dual purpose being the handle. Second, with the head flipped around with the handle on the bottom of the head rubber feet are mounted in the top if the head. Third, you need to be careful nit to loose the special "thumb screw" hardware used to secure the head to the cab. And Fourth, when the head is mounted to the cab with the stock hardware it isn't very secure. It tends to "flop around" because the weight of the head and small footprint of the mounting hardware doesn't provide a stable base. Think of the stock Thunderbass head and Cab behaving like a giant "bobblehead" doll.
The later Guild Thunderbass amps, I think referenced as "1A" models abandoned the original mounting concept and began to install large rubber feet on the bottom if the head so you could just put it on top of the cab. The Thunderstar Bass cabinets actually had four recessions that matched the footprint of the heads feet. I never saw a late model Thunderbass cab but I assume they also had this recession feature.
M