The tether is not a purchase item.
It's just a loop of wire (.027" leadout) wrapped around the engine and secured to the bellcrank.
If the plane is already built, you can get by with a loop around the pushrod.
This is an ancient rule dating back to when a wood engine mount failed and allowed a running engine to get away. The account of the incident is inscribed on a runestone.
Actually: From the AMA rulebook
"The airplane shall have a restraining cable (minimum .027 stranded wire)
that fits snugly around the cylinder barrel/crankcase or may go through a
hole(s) drilled through the engine webs and attaches to the bell crank
mounting bolt."
FAI F2D requirements are similar but more specific about attachment