Mine are always in the last place I look, so that is where you should start!
Usually under something, or stuck in the carpet.
Springs can roll and bounce and will wind up where you least expect them.
Seriously,
you can buy them from many places, Sarco, Lugerdoc, me.
Or go to the hardware store and buy one close in size and fit it.
It is not a spring that is super critical in dimension or strength, JMHO.