I did some research into springs that would fit my Pro-line Powerstroke Scaler shocks, and it turns out these Axial dual rate ones fit. I compiled this table to help me see what I needed, so I thought to add it here as reference.
I think the 20mm springs I currently have will do, and I just need more spring rate on the larger, 40mm ones. I have no idea what rate my current ones are so I've had to buy a selection, just to make sure, so I got green, yellow, and blue from RC Mart.
12.5x20mm
3.6 lbs/in - Super Soft Red http://www.axialracing.com/products/ax30200
4.32 lbs/in - Soft White http://www.axialracing.com/products/ax30201
5.44 lbs/in - Medium Green http://www.axialracing.com/products/ax30202
6.53 lbs/in - Firm Yellow http://www.axialracing.com/products/ax30203
7.95 lbs/in - Super Firm Blue http://www.axialracing.com/products/ax30204
12.5x40mm
2.7 lbs/in - Super Soft Red http://www.axialracing.com/products/ax30205
3.6 lbs/in - Soft White http://www.axialracing.com/products/ax30206
4.08 lbs/in - Medium Green http://www.axialracing.com/products/ax30207
5.44 lbs/in - Firm Yellow http://www.axialracing.com/products/ax30208
6.81 lbs/in - Super Firm Blue http://www.axialracing.com/products/ax30209