Clamps are where you find them.

Clothes pins. And "reversed" they make semi-parallel jaw clamps that are even better. Lead weights for gravity clamps. #64 rubber bands stretched between pencils or dowels make long clamps. And a rubberband between two "T" pins makes a great clamp to hold objects down to a flat board.

You can get the plastic clamps at Lowes or Home Depot . . . or a dollar store.