For two Dorchester moms, celebrating the 375th anniversary of the nation’s oldest public school sparked an idea for something new. Marie Zemler Wu and Sierra Rothberg were so engrossed by party planning for their fête for Mather Elementary last year that they decided to start their own niche venture: Lusterity, an online one-stop party shop for products that are either handmade, sustainable, local or created by a nonprofit or minority-led company. Of course, Rothberg says, “It still has to be an amazing product or service.” Lusterity, which made its first sale in October, offers everything from cake toppers to cheeky napkins, and it’s already expanding. An online questionnaire will soon match planners with Lusterity-approved vendors, and workshops are on the books for December. “Our interest,” Zemler Wu says, “is always in meeting other creatives who want to grow and shape their business in a community with other small businesses.”