Dawn Wells ("Mary Ann") would often answer this question while touring with Bob Denver ("Gilligan"). She basically said that the Howells were responsible for the seemingly endless amount of outfits that the castaways would wear; for example, both Natalie Schafer ("Mrs. Howell") and Tina Louise ("Ginger") wore nurse's outfits when other cast members were sick or if they were assisting the Professor (Russell Johnson).
The entire opening sequence of the first pilot shows Bob Denver ("Gilligan") struggling to carry a large number of suitcases on board the ship (the "SS Minnow"). Supposedly the Howells, being rich and eccentric, chose to travel with a great deal of their belongings, which included many changes of clothes.
It should be noted, however, that during the episode "Lights, Camera, Action!" (Season 2, Episode 7) that Gilligan (Bob Denver) and Skipper (Alan Hale Jr.) are able to salvage two massive trunks from a sunken yacht.
One of the trunks contains a movie projector, screen, and blank film (it belonged to the late Fifi LaFrance and her husband Ricardo Laughingwell, both silent film stars.) The other trunk, however, contained a bevy of old costumes, all of which the castaways presumably used for clothing.
There was also recycling of materials on hand and found material into clothes. The women are seen sewing in first season episodes. Mary Ann and Ginger are shown sewing by hand, while in "The Matchmaker" (Season 1, Episode 25) Mrs. Howell is shown using a pedal powered bamboo sewing machine. Ginger sometimes wears a dress with "SS Minnow" stenciled on it, which she explains she made from Gilligan's duffle bag. In another episode, a weather balloon which landed on the island is cut up for use as fabric.