WE've been many times overall, and several times in Spetember specifically and NEVER needed bug spray at WDW. The most I've ever gotten was like one or two bug bites the entire trip. I suppose we're often eating dinner right at dusk, but we're almost always in the parks after dark. Depends onthe time of year. Sometimes we're walking around at dusk.
Theonly time I think you might need it is MAYBE if you are going to the Mickey's backyard BBQ or staying at Ft. Wilderness. I've only been over there once, and it is a bit woodsy. We went to the Hoop de Doo and stood outside for a while and never got a single bug bite.
As for selling bug spray...I think they have it in most resort gift shops. Resort prices are maybe higher than you'd pay at your local big box store, but not high enough to break the trip budget. (I'm sure the difference in price is under the $15 fee for bags that most airlines charge).
the one thign you MAY well encounter this time of year is a harmless little critter called a love bug. Actually, if yousee them this time of year they will almost all be in pairs, as this is mating season. They don't bite, but they are slow flyers. No bug spray will do any good. Every year there is, IMO, a comical thread about the influx of love bugs.
One more critter, that has been rumored on the DIS is bed bugs. I've never seen any of these either, but it might be wise to check your bedding upon arrival in your room.
