Making your own gimp from the buttonhole silk is definitely most cost efficient.
As for cutting the shape, you can nip a triangular shape into the end in place of buying a round punch. With most overcoats, the extra space is needed to allow for the shank of the button to lay properly in the hole.
Here in the states, silk buttonhole twist is difficult to source in small amounts. Most is sold on large cones like the gimp. (I've only found small cards from vendors for which I need nothing else). I discovered a local hobby shop stocks silk beading thread on small cards, in 2 different weights and 3 colors - white, black and beige. I'm set for a bit as a result. And if I need a special color, I'll spend $3 on a permanent ink art pen and dye the white or beige to get the color I need.
I do miss the days of being able to run to one of six lical fabric stores to buy what I need!