If you aren't paying money you might want to be a bit more kind with your words. I would say something like, "I don't have any money to offer, but I was wondering if someone would be kind enough to make me a simple banner." There are plenty of people willing to do it, but I don't think offering to "vouch" for someone will go very far. I realize that you aren't asking in a rude way, but I think you would have better chances just asking nicely.
