Hit your local Gunshow. Buy a few Chinese Assisted, don't pay a lot 5 or 6 bucks each. I bought a few before I found a design I liked. I ended up finding one with a single leaf spring and extra stop pin. They all work on same principle the spring hold it in place till it reaches certain point and spring reverses and opens the blade.
Stay away from coil springs and weird looking springs. Custom made springs can get expensive. Look for leaf spring designs. Leaf Springs are easy to obtain materials and make.