Not sure where you can get, but 2 things:
1) Attenuators are only for full-tube amps. Valvestate and the likes do not count. (I think Vox Valvetronix does though, it has an in-built attenuator that I find really lacking.)
2) You need to match the impedance of the amp with the attenuator. Would be best to get same impedance for the amp, attenuator and speaker i guess.
Cheers
Raymond