Hi there and welcome to the site. Do a search for uk suppliers as it gets asked quite a bit. LePrevo is who I have always used, Bowstock, Barkers, Tandy, Abbey saddlery (account needed though).
As for the sizes, it does depend on size of the sheaths and pouches you plan on doing so best bet is to get your tape measure out and figure out some sizes. Most leathers are sold by the sq/ft so keep that in mind.