Where Can I Find an Inexpensive Bike Stand Clamp
If you are looking for just the clamp, and want something produced commercially you are going to have a hard time finding something inexpensive (at least by my budget).  A Google search for Park Repair Clamp turns up a number of options but they are generally $100 and up.  You can pick up a decent stand for that much.

The best deal I could find was a brand I'd never heard of:

Sette Workbench Stand (or this search)



Your other option would be to build your own stand:

DIY Bench Mount Bike Repair Stand

Work Bench Repair Stand with Angle Adjustment

Hopefully you get the idea...