Different tile cutter manufacturers use different diametres and bearings. Sigma scoring wheels, for example, are different from Montolit scoring wheels. You'll have to get one which fits your cutter specifically. You should be able to find them at any shop which sells the brand of cutters you use.
As for wet cutter discs... I'd recommend ********* G2 for most ceramic and porcelain tiles. They cut cleanly at good speed. Most on the forums will say "Marcrist" though. Other good brands are Rubi, Montolit and Sankyo, to name a few.