The drinking age in this province is 19.
The drinking age in the province of Ontario is 19. and in the State of New York its 21. That means that consumers over the drinking age may legally buy and consume alcohol. If you were born after 1999 (if drinking in Ontario) or after 1996 (if drinking in New York) you will not find anything of interest here for now.
Because Niagara Falls sits on the Canada-US Border, there is also some competitive shopping going on (lower taxes here... better selection there... whatever) where liquor, beer and wine may be bought in one country and transported to the other. Keep in mind that if you are an American under 21, you can legally buy alcohol in Canada, but you are considered too young to bring it back to the United States. Those transporting alcohol in either direction need to be aware that there are customs duties and (more significantly) state & provincial excise taxes sometime due on any alcohol that exceeds your permissible personal exemption.
Whether you are shopping for yourself or for a large party, Niagara Falls offers a wide variety of beer and liquor stores. On the Ontario side, you're stuck buying from your neighbourhood Ontario Liquor Store or the Beer Store. The Beer Stores are operated on behalf of the large breweries (so you may not always find the micro-brewed brands) Liquor stores also sell beer, but usually international beers and very popular Canadian brands.
Around the province, you can buy beer or wine, in stores run by the makers/wineries themselves, typically a micro-brewery (like the Upper Canada Brewing Co.) or one of the many fine Ontario wineries. So take the time to find one that has the items you like or is willing to special order them for you.