I don't think horizontal replacements would be available. But, if both sides bolt on the same way, you could fab up a pair yourself. You'd just need a drill, hacksaw, files...and plenty of time.
It would be easy to have a machine shop make some too, but that wouldn't be cheap- it would probably cost way more than the frame did.
Of course with a $20 chain tensioner you'd be ready to go in 10 minutes!
I'll agree with Live Wire. I would think that tabbing some up wouldn't be particularly difficult. Perhaps you could fine some weld-on dropouts that would have enough material to cut off the weld areas and shape/drill appropriately for your frame.