Does it have to be a "dog" bag, or can you just get a cute messenger bag and use it as a dog bad? That is what I would do! Mickey on YT has a messenger bag that his yorkie rides it, but I don't think it is technically a "dog" bag.
Go to
www.aol.com/shopping. You can type in what you are looking for and it will bring up a whole list of retailers that sell that product and pictures. It is very useful
Good luck in finding a cute messenger dog carrier!