Most cocker and older spider valve springs work fine I don't if you still can get the but back in the day you could get a spring pack. It had three springs a red one = hard a blue one= mid and a yellow one = soft. It was like $5.00 at the time.
Anonymous Coward is right about crushing the top of the spring to. As seen here
The spring will stay on the cupseal which makes thing a lot easyer down the road.