Needing to replace "pistons" for raising and maintaining the open door of my shell covering for pick-up. where can I get them?

Reputation should be part of your assessment. I have nothing but praise for napa in Bend. The one across from Wilco

The price range can be so wide on these. I would check internet prices before I bought at a store locally

These are actually pretty special. Don't just get the right size, you need to consider pressure too. They are relatively expensive.