Ginger made herself at least one or two dresses from the sails and canvass of the Minnow (they even had the boat's name stencilled on them).
She might have also made some clothes for Mary Ann. And in a couple of episodes trunks washed up in the lagoon that had costumes and stuff in them... they could have altered some of that for new clothing. Skipper, Gilligan and the Professor did seem to wear the same clothes every day.
Can't explain why the Howells had so many luxury items, and the Professor so many books. You could explain Skipper and Gilligan having more stuff because the Minnow would practically have been their home... but ironically they seemed to have the least amount of possessions.