Allparts is the only distributor of ABM Mueller in the US.
http://www.allparts.com/abm
If your looking to replace your nashville bridge, look into Faber's options:
http://www.cvgshop.com/Faber-Parts/c4/index.html
As mentioned previously, I too agree that coupling is a huge factor for tone. It really all boils down to how well all of the parts work and fit together. I have installed both the Tonepros and Faber locking nashville bridges with aluminum tailpieces on the same guitar and I much prefer the Faber.
I replaced the all the Schaller hardware that was installed and experienced the following:
I installed the Tonepros set first thought I would get more from it, but it is essentially just a standard Gotoh bridge (Gotoh owns the Tonepros Brand) with small locking hex screws but 3 times the price. It just seemed like the locking screws just held the bridge and tailpiece in place rather than really increase the coupling. The guitar did get a slightly brighter and had a small increase in clarity both acoustically and plugged in which I attribute to the aluminum tailpiece.
With the Faber, I installed the whole Tone Lock bridge and tailpiece kit with their steel bridge studs. Huge increase in the acoustic volume, resonance and clarity. I could instantly not only hear a change, but I could also feel it. I could really feel the vibrations of the strings throughout the whole guitar which was something that was lacking with the previous two sets of hardware. With the way the Faber stuff is engineered, all of the pieces actually lock together extremely well increasing coupling dramatically. Another plus side to the Faber is that their tailpiece inserts allow you to raise it up off the guitar to avoid the strings hitting the back edge of the bridge yet still have it firmly locked down. Overall I just feel is a very well made system with some great innovations.
Sorry for the glowing review, but it's rare to have something work so well and and really make such an impact tonally.