Well you could try toys r us in Hove or there is a shop on Trafalgar Street, the one on the way up to the station from St Peters Church, that has loads of kids costumes on a rail outside.
Oh and the entertainer in town sells light sabers. Think Argos do too and I'm sure toys r us would. I know I got DS2 one a couple of years ago for a fiver. Beware though they are quite deadly in the hands of a young one.