In the end, I was unable to find the right springs for my needs: ones that had the right strength/resistance and diameter. I decided to make my own springs, so that I could make them to my exact specifications, using the music wire below:
To create the springs, I created a wooden jig, after some trial and error--and eventually got some great results! To research spring-making, I referred to this immensely helpful website: How to Make Springs. The spring-making jig I created was different from the designs on that site, but I was able to leverage many of the concepts they described. I'll go into more detail on my particular jig, and how it works, in another post!
Here is a video of Gertie jumping for the first time, and the delta kinematics at work: