A good alternative that's easier on the wallet is the Maxon AF-9. I have an old one and have compared it side by side with a friend's vintage Mu-Tron and they are remarkably close. I would avoid the recent Ibanez AF-9 as it has a different circuit. (Maxon used to make the Ibanez pedals). The story I was told is that Maxon licensed the Mu-Tron circuit, only running it at 9V rater the 18V. The controls are identical.
The differences I found between the Mu-Tron and the Maxon is that the Maxon is harder to trigger and the Mu-Tron sustains a little bit longer.