One possibility is to ask around the sewing stores in your area about people who embroider patches. Small orders are generally more expensive, and if you are looking for something that is very reflective to use as the backing, you'll have to provide it and pay up if it breaks their machine, but it might be an alternative.

The other is just to get a piece of 1/2 or 1" red webbing, and sew a red cross onto the bag. Or if it has a shoulder sling, get one made up with red webbing. (Not hard, doesn't even need sewing if you use tri-glides)