(The manual dimmer should work without any computer!)
You could use a rotary encoder for that purpose!
Basically the rotary encoder is a knob that, for every "tick" you give it, it returns a different value to your Arduino. With a bit of programming you can sense the direction of the turn, if it is positive then you lighten up the LED, and vice versa.
I used that to control a RGB led, it works very well! here some more information on the rotary encoder and the code needed to use it.
I add another answer because I think I found something. After a few days searching on various sites that sell arduino stuff I found this http://www.dx.com/p/8-channel-5v-relay-module-extension-board-for-arduino-avr-arm-51-140703 that might be what you're looking for. One of the customers in the review states it can be used even without Arduino.
I know it's a relay, not a dimmer, but maybe you could add something to your circuit...