Leave it to "the intern" to tell me about an awesome store on Newbury St. (and also online) called 1154 Lill Studio. You can design your own purse, wallet, clutch, bag, or even pillow. Here are a few that I created using their online design tool.
I think this would be so much fun to go into the store and pick out my own custom bag. They're not cheap. They range from about $30 - $210 depending on the size of the item. Check them out. If anything it's a fun hour (or two or three) playing around with some designs on their site.