Thanks for contacting us. Are you sure you have the S900-33? Is the outer tube 3/8"? Make sure you're not measuring the internal spring when determining the diameter, you only want to measure the outer tube.
The reason I ask is because no Blue Tip was ever made for this size. There is a Green Tip, but it has a range of 14 through 26 pounds; this is covered by both the White and Yellow tip we already provide for this size, which is why we don't carry it.
Assuming you do have a Series 900, then you might be looking at it the wrong way. I recommend getting the exact weights of your current sashes and seeing if the White and Yellow tip options would work.
