Not necessarily the "best" place, but a useful source of fasteners is Screwfix
http://www.screwfix.com/app/sfd/cat/...xHome&ts=49724
They have expanded the range of stuff they do and now stock a decent range of fasteners, including a selection of stainless stuff (nylocs, dome nuts, capscrews, studding etc) and zinc plated flanged nuts like the sprocket fixings. Pretty sure the sprocket nuts are a std M10x1.5 thread. The S/fix ones are grade8, which should be OK for most purposes. The plating isn't usually very salt resistant though!
Look in the "bolts" section of the Screwfix site. Note they have a min delivery charge! (I have no connection, but have bought stuff from them)