I personally just went through this hassle of finding 5M pots in Europe.
These guys have it, but only as dual pots. These are very high quality pots, http://www.banzaieffects.com/Stereo-c-471.html
Look at the end of the list. You have a choice:
1. take the 5M one, and simply don't use the other side,
2. or take the 2M5, and put both pot sides in series to get 5M total (slightly cheaper, I grabbed this one).
I also spent quite a while browsing through the suppliers, until I accidentally stumbled upon this site (without seeking the pot this time) www.uraltone.com
and what's best for me, they're in Finland
PS. They also have a nice tube diy guitar amp project for sale, check out Uraltone Jr. Maybe someone could make a cool case for this?