Seidio the undisputed brand for holster cases has a nice one out for the HTC Thunderbolt.
The holster for HTC Thunderbolt pictured above has a swivel belt clip which you can attach to any belt or purse for easy access to your mobile. A great way to display and protect your cool Android cell without a case on the phone, as many like to carry theirs naked. This one is top notch and features the following.
* Unique spring clip lifts for quick access and easy removal
* Adjustable swivel belt clip with 7 rotating angles
* Felt liner and face-in design protect your phone from outside elements
* Easy access to connectors and function keys
* For use with HTC Thunderbolt
* For use with non-cased device only
* For use with: HTC ThunderBolt
We found the holster for HTC Thunderbolt online via mega-cool retailer Amazon.
p.s. You might also be interested in the cool HTC Thunderbolt armband case from Cell Phone Shop.