AFAIK, our local Scout Stores have never offered such an event. I don't think they have room or qualified staff for such an offering.
There are other places that offer some good MB classes. The planetarium does Astronomy, the zoo does Bird Study, Insect Study, Reptile and Amphibian Study, Mammal Study, Nature, Environmental Science, etc.. The fitness center offers Swimming, Lifesaving, Personal Fitness, etc.. Many other opportunities exist.
The existence of MB classes is not inherently bad, although I have seen both bad and good. A good class gives boys access to expertise and equipment that they may not otherwise have. A good instructor can lead discussion, and present needed information while making sure to include all participants in the discussion and learning. A good class will be sure that each participant fulfills each requirement as it is written. Each participant will 'pass' the class based on his own completion of the requirements, individually, not as part of the group. Most MBs can't be completed in a single class session. Most will require some work outside of 'class' (prerequisites, homework) to complete the MB.
Over the years, I have instructed 'classes' in varied settings for hundreds of scouts. Those who do the required work, get a signed blue card. Those who do less than the required work get a 'partial'. Some of the boys who get a 'partial' will contact me later to complete, others won't. A class where every boy completes everything is a rare occurrence.
Eagle Scout 1987
OA Vigil Honor 1986
Fox - WE7-590-05-2
Currently - Troop/Team/Crew Advancement Chair & Dist Webmaster
Previously - SM, MC, CM, ACM, ADL, ASM, COR, Dist Camp Chair, PL, SPL, Scribe, Songleader, JASM, OA Chapter Officer, ...