Most suppliers of British motorcycle spares stock reasonably priced Whitworth socket sets and combination wrenches, usually of asian manufacture. Maybe try Moore's Cycle Supply, West Hartford,Ct.(has website), British Marketing,Laguna Niguel,CA, or Fair Spares America, San Jose, CA. I have a set of Koken sockets I've used for years; not pro quality perhaps, but fine for hobbyist use.
Regards,
Wes Gadd