You can try calling directly to Swift or hit up their website. 1-877-697-9438 http://www.swift-sportoptics.com
I would bet that they'd get you set up with no problem.
I know Scheels in Fargo is a Swift dealer, I'd suspect the entire chain is so try the Sioux Falls store.
I've bought a lot of mine at Sodak in Aberdeen or Kone's Korner west of Castlewood.
I know Sodak used to carry spare sunshades because I got a replacement from em a few years ago. Never had to replace a turret cap yet...knock on wood!
I don't have my thread pitch gauges at the house so I can't measure mine tonight. Maybe tomorrow if I remember to take a cap to work.
If your scope has a red ring on the objective bell then it's a Premium scope, if it's got a gold ring it's an older Premium. If no ring, it should be the Reliant line.
The Reliant is the "bargain" line but they are still darn good scopes!
You should be able to read the model number on the ocular bell of the scope too...unless it's rubbed off.
The caps do interchange between the Reliants and the older Premiums. I don't have a red ring Premium but I'd guess they're the same size.