Some basics first:
- "racing bars" doesn't really tell us anything. Plenty of different bars are used in racing. I'm guessing you mean drop bars.
- "standard size clamp" is equally pointless. There are several different standards still holding on to some sort of following. I'm guessing it's the latest, 31.8 mm one that you're getting hung up on.
Now to specifics: 25.4 and 26.0 mm is readily available at store.velo-orange.comhandlebars.htmlOpen ↗ for instance.
sheldonbrown.comindex.htmlOpen ↗ Also carry those, and a rare 25.0 mm to boot.
Unless measured with a caliper, these would look just like 24 mm bars for all but the most skilled visual inspectors.
I've read about 22.2 mm drop bars, but I've never seen one.
If that's what you've got, I'd suggest getting a new stem to go with your new bar instead. It'll probably be a lot easier.