You can buy your own seatbelt extender. Technically it's not authorized, and a sharp-eyed FA who knows the rules will not let you use it. But 95% of the time, you can get away with it.

Last week I was on an Eagle jungle jet and the seatbelt was so short that anyone over 220 wouldn't have been able to fit in it. That's the most egregious I've seen. DOT should really require a minimum length for seatbelts.