
Other than the obvious of going to crufts to discover dogs(any of the 4 days) or on the day Boston Terriers are shown(Saturday 10 March) which you would see a fair number of them.
My other suggestion would be Blackpool Championship Dog show on Fri 22 June where there will be some there. There could some at other shows which is a good place to meet a breed your interested in...even if shows are not what your interested in.
I have a few friends who own/breed/show Bostons but I don't know a great deal to be able to help you on them as a breed.
As has been already suggested it would be a good idea to contact secretaries of Boston breed club/s who should be able to help you more