I doubt you'll find a clamp on stop for your OS tubing, but all is not lost.

There's a repair kit, which consists of a pair of bosses connected by a threaded rod which goes through the frame to hold them in place. Offhand I don't know the source, but I'm sure others will post pointing you in the right direction. This would require cutting the opposite one off, and filing flush to prepare the frame for the replacements. I suggest that you reinforce the job by use an adhesive, or something like JB Weld in addition to the bolt to secure them, so there's no tendency to rotate or work loose.

If you cannot find the kit, you might source a boss, cut off and file the old one flush, drill out the hole slightly and use a riv-nut to attach the replacement. In this case I strongly suggest bonding it to spread the load, otherwise it's likely to rip out taking the Riv-nut with it.

Here's the source info for the repair part I was describing Hopefully you can use it to solve your problem.