Where do you get scrap wire for making pigtail connections e.g. when wiring a switch?

As you're doing other wiring, you're bound to cut off bits that are 6" or longer. Keep 'em somewhere handy and use these when necessary.

Just remember, it's OK to use #12 for pigtails on a 15amp circuit with #14 wire, but IT IS NOT OK to use #14 on a 20amp circuit that requires #12 wire end-to-end. It's pretty easy to tell them apart, even if you'll pulled wires out of the cable and have just the bits of wire laying around.

If you don't have scrap NM/UF handy to shuck...

While the typical way electricians make pigtails is by "shucking" the jacket from NM or UF cable of the appropriate gauge and using the wires inside, this isn't the only way to do it. Many hardware stores and electrical supply houses will sell THHN by-the-foot for a small additional charge atop the per-foot price of the wire, and appropriate gauge THHN (either solid or stranded) can be used for pigtails as well. (Note that stranded wire doesn't mix with backstabs, not that backstab connections were ever a good idea to begin with!)

Many stores will cheerfully sell you NM or THWN wire by-the-foot. I typically buy 2 feet of it, shuck the sheath off the NM, and then cut it into either three 8" or four 6" lengths, depending on how long I want my pigtails.

That's when I'm working far from where I do electrical, obviously I have a glut of wire here.

If your preferred store won't sell by-the-foot, that's not an electrical problem, that's a shopping problem.

If you had a good relationship with a proper electrical supply house, I would expect them to disappear into the back, and come back with a few scraps, wave hand and say "don¡t worry about it".