The biggest challenge with replacement pots finding the exact mechanical specs such as shaft length, shaft diameter, type of shaft finish to accommodate the proper knob - things like that. 1Meg pots are available but finding the right one with the proper taps for Loudness, etc, makes it harder to match, too.
How about some more details like the model amp involved, etc? Makes it hard to make suggestions without specifics.
Cheers,
David