Harbor Freight is the best place to buy clamps .

Or Amazon will often have good prices on them .

A lot of times you can just screw down a scrap of wood to the base you make the jig from to act as a clamp / wedge .

I often make jigs when scratch building but never for OOB kits  ... that I can think of anyway .

Mostly task specific jigs  , so I just glue the elements in place .

some in these threads :  https://www.scalemodeladdict.com/threads/himmelsfalke-ma-k-falke-xl.13485/

                                       https://www.scalemodeladdict.com/threads/this-other-thing-im-building.13569/