Evidently, it’s impossible to make a spring both good and noncorrosive:
I found a suitable (i.e., good, but rust-prone) spring in the Big Box o’ Medium Springs:
Unlike the repair for that sprayer, this spring turned out to be long enough to work perfectly. I have no idea how long I can keep this up, but … at least I’m now keeping pace with the failures.
The Smell of Molten Projects in the Morning 
Seems like 6Al4V ought to work just fine.
And hey, turns out you can buy it on Amazon!
Missed my chance: “Currently unavailable. We don’t know when or if this item will be back in stock.”
I’ll put it on my Santa wish-list… [grin]
How about a plastic spring?
for example, http://www.advanexusa.com/plastic_springs.html
This scares me right off: “E-mail for the latest catalogue and prices!”
My rule of thumb is that if I must ask for their prices, I can’t afford them… but if I were making a bazillion sprinkler heads, a non-corroding spring would be a nice selling point!