Mcalpine Sink Traps?

My Kitchen Sink has a Mcalpine Trap fitted, its about 10 years old and now when emptying a full sink, Water leaks from the top part. I"ve unscrewed it and theres a rubber washer with a pin inside. can I buy a new full trap? I don"t know model number. but I hope it is a standard size? what price am I looking at?