Any electrical retailer (not the Curry/Comet type), some DIY stores will have RCB's -- or the more specialist stores such as RS Components and Farnells.
The type C and type D are what is called "motor rated" so they don't trip when a high inrush current is demanded, (leccy motors need a high current to start turning but only for a short duration). Did you get random tripping of the failed RCB when switching on an appliance?.
If you plan to use a vacuum cleaner/drill/lathe/fridge/hairdryer on the sockets protected by the failed RCB go for C or D type. D are more tolerant than C.