This is the type of clamp I think those who mentioned Harbor Freight are talking about:
They're nylon, and HF sells them in a tube of a dozen or so, in different sizes.
If you get these, do be aware that the gripping surface might need some help to grip and not let the clamp slip off. For example, when used to clamp the upper and lower halves of a wing, the clamps have slipped right off from the force of the spring and insufficient friction to the piece.
I've put masking tape on some of mine; others have mentioned gluing bits of sandpaper or other abrasives.
I still use clothespins for most clamping jobs, or rubber bands.