TrunkTracker III, IV, and V are all marketing terms for Uniden's line of products and are not inclusive of other options on the market. If you are looking for a Uniden product, then you want to look for anything that is TrunkTracker IV (P25 Phase I) or V (P25 Phase II) (the higher the number, the "better"). Any scanner marketed as digital will function for SIRN. SIRN is operating as P25 Phase I, so just about anything on the list I am attaching below will follow it.
A consideration to make is that a P25 Phase II scanner is backwards compatible and will properly follow the system you are looking for. It is also relatively future proof (for the time being) in the event that SIRN upgrades to P25 Phase II. If the scanner is not on this list, then it cannot do P25. You might also want to visit the West Virginia Radio Discussion Forum
to look for discussions regarding SIRN and whether or not there are plans to upgrade the system to Phase II or for recommendations on scanners. It's possible that a P25 Phase I scanner will keep you going for a long while.
Current P25 Phase II Scanners:
Special Case P25 Phase II Scanners:
The following model is discontinued but may still be found online. Will eventually be resold under a different name in the future.
[Future Whistler WS1080] (Handheld)
Current P25 Phase I Scanners:
RadioShack Pro-651 [Same as the discontinued RadioShack Pro-106
& Future Whistler WS1040] (Handheld)
RadioShack Pro-652 [Same as the discontinued RadioShack Pro-197
& Future Whistler WS1065] (Base/Mobile)
Discontinued P25 Phase I Scanners:
Older Discontinued P25 Phase I Scanners (Lacking 700 MHz):