I cannot find any website that could be trusted. What is the current average price of diesel in Toronto or the East Coast?
There is no official, central listing of fuel stations and prices in either Canada or the U.S. I'm not sure there is for any country where the retail market is not dominated by a state monopoly.
For Canada, as noted in Jim's answer, your best bet may be GasBuddy, a crowd-sourced website where people submit current prices posted for individual stations through their mobile app or website. The Canadian Automobile Association sources its gas price listings from GasBuddy.
A search on Toronto shows that as of July 3, 2018, diesel is selling retail at around 1.31 CAD per liter, with prices reaching as high as 1.39 in North York and as low as 1.25 in Oshawa. The usual caveats with crowdsourced resources apply, however; a large number of stations even within the city of Toronto have not received recent updates. Moreover, the updates are only as accurate as submitted by essentially anonymous strangers, who might accidentally miskey an entry—or who might deliberately submit erroneous entries to sabotage a competitor (GasBuddy has some mechanisms to fight this, but it's the same problem that occurs with Google Maps and Yelp and the like).
I'm not sure what you mean by East Coast; that term is not very common in Canada for referring to "the Maritimes" or "Atlantic Canada." It is a common term for the Northeastern U.S. or the entire eastern seaboard of the U.S., however, so if you are interested in the U.S., note that there are more GasBuddy users there, which may contribute to more reliable results in more locations.
Unlike CAA, the AAA Gas Prices website and mobile app rely primarily on a proprietary service, OPIS (owned by IHS Markit) which conducts a daily survey of stations. OPIS is also the source for fuel prices as offered by MapQuest, Automotive.com, GEICO, and many others.
I link AAA since it is broken down by metropolitan area rather than by county, as it is not always easy for travelers to know which county they are located in. You must drill down first to the state level, then select the local area, although you must check each state if a metro area crosses the border. In Philadelphia and its Pennsylvania suburbs, diesel is averaging about 3.56 USD per gallon, whereas it is averaging 3.14 USD the New Jersey suburbs.
This is not my site, but it was founded by a local which is how I found out about it: Gas Buddy has prices in most Canadian and U.S. cities, though the prices are most current only in the big cities.
Also, "Toronto" and "east coast" are two very different things. Gas prices vary wildly from city to city and province to province (assuming you mean east coast Canada; it's even worse if you want to include the east coast U.S.). Expect to pay a lot more for fuel in St. John's, Canada's easternmost city, than in Toronto.