What do I use to imprint the sides of a purse cake done in buttercream. I was thinking of maybe using a dowl??? I'm not sure, this is my first purse cake. thanks

Depends on what ya want to 'imprint' on it. How do ya want it to look?
Like a pattern of some sort?
I have some polymer clay impression mats that I use to imprint on fondant items, I am sure they'd work just as well on bc.