"Where can I buy a ticket to get around/go sighseeing london by bus"
"Where can I buy a ticket to get around/go sighseeing london on bus"
< Answer to other question now in other thread. Cagey, moderator. >
Your thread title is 'On or by' The correct preposition here is 'by' which we often use when talking about means of transport: to travel by bus, by bike by train.
< Edited to remove comment that is no longer needed. Cagey, moderator. >