If we are caught on the hop, we will either create our own using knifeless tape or do as Neil says and buy the kit in from a trade supplier. Not often I require them but when I do I source from William Smith.
I have created my own templates in the past, but think it is only any good if you are doing lots of the same vehicle, due to how long it takes to get the template accurate.
You then have to stock another grade of vinyl that isn’t always cuttable on the plotter. Like the others have said, ordering one in is more convenient for me and less hassle. granted my markup isn’t as big but still.