Do you sell replacement latch springs?

I'm afraid I don't know what it is you need, exactly. Is this a spring for a mortise latch? If so, these are rarely (if ever) sold separately and in most cases you will need an all new mortise.

For clarity, would it be possible to see a few photos of the latch in question? That will give me a much better idea of what you're looking for and whether it's something we can supply.