Protek supplies offer a cheap spring for the Meteor, and another one for the larger BSAs.
The springs are claimed to be suitable for several other guns too, so I'm considering trying some as my collection of old springers has grown bigger than my supply of spare springs.

But the protek springs seem to have quite stout wire, at 3,55mm for the Airsporter spring and 3,5 for the meteor spring.
Has anyone on here tried them?