In Dan Beard Council (Greater Cincinnati) we are fortunate that somebody took the time to compile a very thorough listing. Organized by districts, merit badges, anyway you want it really. I suspect it simply takes somebody willing to put in the time and effort. Easy enough to maintain once it is first in place.
This is a recent change. When my eldest son first joined a troop, the list was outdated, difficult to navigate and held behind a password. Users were told that the list was not to be shared with scouts, but MBCs could be selected by the SM or Advancement chair and given to the scout. Then the password-protection was dropped but the list was not updated. I actively recruited MBCs from our troop, acquaintances, anyone who I thought would work well with youth and had the background to mentor a merit badge. Most applications were not approved. Not rejected, either, they disappeared into the black hole. My MBC app took my DE personally walking it through the process for me to gain approval the third time I tried. Council finally has its act together and the list is as you described, and a joy to use.
I have long suspected that the difficulty in getting MBC information led to troops creating their own corp of counselors and relying heavily on summer camp and merit badge challenges to help their scouts earn merit badges. Now that understandable response is undermining the process itself. It's a shame, really.