I use old broken bandsaw blades. If they are too long, you can cut them off with an abrasive cutoff wheel on a Dremel or angle grinder, or score them and bend them back and forth a couple times. Problem with offering more specific advise is there are lots of different steels and heat treatments used on different bandsaw blades.
Note that I have two different stretchers for my bowsaw. I think one is 12" and the other is 14", to match various saw blade lengths (e.g., the hacksaw blades I usually keep loaded in the bowsaw). If you radius the ends of the stretcher (convex) and put a matching concave radius in the arms, it makes it easier to accommodate small differences in the length of your blades. I use the largest drum on my spindle sander for the arm radii.
You could also build a small bowsaw designed to hold coping saw blades, similar to the one by Brian Buckner on BugBear's archived web page.