There's a Long's Drugs in Lahaina you could stop at en route to Kapalua. Also, closer to OGG there's a Long's (Maui Mall) and Wal-Mart (near Dairy Rd). You could stop here after arriving from the airport.
BTW, Long's and Wal-Mart are also probably the better places to buy your souvenirs; much better prices, and that's where many locals stock up on gifts to bring to friends/family on the mainland.