In 21st-century English, buses is the preferred plural of the noun bus. Busses appears occasionally, and dictionaries list it as a secondary spelling, but it’s been out of favor for over a century. This is true in all main varieties of English.
After bus emerged in the 19th century as an abbreviation of omnibus, buses and busses (the logical plural of buss, an early alternative spelling of bus) vied for dominance for several decades. By the early 20th century, though, buses was the clear winner, and it has steadily become more prevalent. Today, buses appears on the web about 15 times for every instance of busses.
This ngram graphs the use of buses, busses, and omnibuses in English-language books and magazines published between 1800 and 2000:
Buses is far more common than busses in edited news sources that publish online. Here are a few examples:
New vehicles and routes and added frequency have helped draw people on to buses this winter. [Financial Times]
In many cities around the world, there are simple established rules for riding buses. [New York Times]
Aucklanders will be waiting until November at the earliest to use a single card on buses, trains and ferries. [New Zealand Herald]
But it’s easy enough to track down a few examples of busses in the wild:
Five school busses were required to transport Pueblo’s Heaton Middle School’s 180 band members and their equipment Saturday. [Canon City Daily Record]
Putnam County will provide shuttle busses to transport persons between the parking area and the funeral home. [Press release quoted on Patch]