As far as I can tell, RedBubble requires a DMCA takedown notice; they describe how to send one here: [link] Egophiliac wrote about dealing with this situation last year in this journal entry: [link] Specifically, she says she gave them a PO Box for her address, and listed her name and her dA name for her "digital signature", and that worked, though I can't swear they haven't gotten more strict in the intervening months.
Good luck! I really want to see that art thief go down.
You've probably already figured this out, but I somehow derped the link in my original comment; this is the t-shirt s/he made from your drawing: [link] Sorry about any confusion. :/ A couple of other sellers that I noticed offering stolen work on RedBubble have either taken the offending works down or have vanished altogether, but it looks like FlutterPaw's chugging right along for now.