Depends on the MP, and the retail outlet. Where I am, only Yamada Denki stocks MPs aside from the specialty stores in the city. If you're in a major city you might have some hobby shops around, if not then you'll have to try mass retail or take a trip to Tokyo/Osaka/one of the other major cities.
Either way, MP Soundwave was gone the morning he was released. Soundblaster lasted a bit longer but still vanished after a month-odd. The odd MP Smokescreen was still floating around for weeks after it was released.
I'd imagine Bumble would go pretty quickly.