In your title, you said "a bus" but in your thread, you said "the bus". The choice of article is appropriate in different contexts.
Which is the bus for/to London? (You are looking at several buses and need to know which one to get on.)
Where can I get a bus to London? (The only information you are giving is that you want to travel by bus, rather than by train or plane or car, so you need the location of the starting point locally for a bus going to London. The answer to this question could be "From Manchester or Liverpool or Brighton or Glasgow or just about any city in the UK.")
Where can I get the bus to London? (You are in a location and you know that somewhere - hopefully nearby - is the bus stop from which you can catch the bus you know goes to London. You just don't know where the bus stop is.)