I would be writing, calling or e-mailing the company that sold them or the manufacturer. You are not going to find parts from other sources. Google should find you where Tranz X is. They may even send you the parts or a new handle bar because they don't want the liability of that part breaking and you getting hurt. Roger
Could you just replace the button lock cam lever with a non locking lever from a qr seat post clamp and then seek alternative means of keeping it from opening, starting with keeping it tight?
I did replace the broken one with a normal one, but they don't tend to have as much a curve on them than the supplied one, meaning they don't curve around the handlebars and don't lock up as much. Keeps coming loose.
I'm trying a couple of things at the moment. Will report back when I get some results
These folding handle bars are very interesting.. I gather you have added them to a bike as opposed to having purchased a new bike with them on.. It appears that they would make a bike narrower.. pictures?
Another alternative that I have seen recently for making a bike narrower is a stem that unlocks and turns the bars 90 degrees. It seemed simple and safe.