The more popular brands have a few throttle controllers. They connect to a USB port, so they can be used with a separate joystick.
For example:
CH Pro Throttle (Single, slides, HOTAS style)
CH Throttle Quadrant (6 levers)
Saitek Throttle Quadrant (3 levers, designed to fit together so you can buy two for 6 levers)
Saitek TPM (Cessna style)
If you want rotaries, something like a Saitek X-55 could work(Has a USB throttle).
It's an expensive bit of kit(especially if you'd only use the throttle), but if you're really looking to get into the hobby, you could consider buying a HOTAS setup(Joystick and throttles together) like that.
CH also has a stick to go with its throttle, and Thrustmaster makes the famous Warthog.