You can buy a decorator's tool from NPC Architects. They spawn in Carpentry shops, like this one in Moonglow.

This may seem silly, but I can't tell you how many people have asked me how to work a dec tool, or who I find out have three dec tools, one for each function, because they do not know that you can acess a menu and change your dec tool an endless number of times.

It's easy. Just double click the tool and then target the tool itself with the cursor. A menu will pop up and you can select raise, lower, or turn.