Cwnco, unless you are only looking for light civilian birds (and in that case, Jim got you some good info), there are absolutely tons of biplanes kit on the market nowadays... All shapes, all sizes, all scales. Hasegawa has some really expensive and exquisite large scale biplanes (Wright brother's Flyer, Fokker Dr.I), and at the other side of the scale spectrum, you can find 1/144 WWI fighters as well. In between, you can chose from Revell, Hobbycraft, Airfix, Frog, Matchbox, Hasegawa, Roden, MPM, Aeroclub, Sword, Azur, Heller, Monogram, William Bros., Esci, Italeri, Eduard,... (to name but a few). I'd stay away from the older Airfix, Frog and Revell kits but would definitely recommend the Eduard and the Roden kits.
That's without even mentionning what's available in vacuform and resin kits...