Make and model keyboard information would be helpful. Age, condition?
If an older keyboard you might find a similar model or even your model at a Goodwill store or other such place.
Or at yard sale. Ask around, friends and family.
Find the applicable keyboard. Take the space bar apart to 1) learn how to do that, and 2) salvage a replacement spring.
Failing that maybe just try a couple of springs via old, unused, random keyboards. Might just fit.