There are two methods...
you load a fresh game and look at the decals and get their links and names. copy and past them one by one to your save.

the other methode requireyou to edit the save file.
go to
%userprofile%\documents\my games\Tabletop Simulator\Mods\ and look there for 1301493206.json
open it in an editor your choice.
scroll almost to the bottom looking for "DecalPallet".
now mark all the stuff between [ and ] and copy it.
now go to
%userprofile%\documents\my games\Tabletop Simulator\Saves\ and look there for your save file.
open it in an editor your choice.
scroll almost to the bottom looking for "DecalPallet".
should look like
"DecalPallet": [],
and between the [ and ] you have to paste all the stuff you had copied.
save the file and load it into TTS.

How can I save my save file? I dont see any "save file" folder

if you save a game in TTS it will appear under %userprofile%\documents\my games\Tabletop Simulator\Saves\