My understanding is that there are only five true sources of sour in the making of cocktails: lemon, lime, grapefruit, yuzu and, rarely, green apple. I always have lemons, limes and a grapefruit in the fridge. People swear organic lemons are best but I’ve never really noticed a difference. The price of citrus has skyrocketed so I don’t think the cost is worth it. Chinatown is where you will find the best prices.
Meyer lemons, available at high end shops like Harvest Wagon in Toronto, are sweeter than regular lemons, and shouldn’t be used interchangeably with the regular variety. Sanko Japanese shop on Queen West gets yuzu citrus in every week but they’re crazy expensive. That said, the floral bouquet that comes off this fruit makes it worthwhile. Citrus suppliers in the US stock other seasonal oddities online like bergamot oranges and rose lemons. All sound fun but getting them into Canada is really tough.
I should also mention that I recently came across a cocktail that utilizes vinegar (white balsamic) as the sour. It’s risky but in this drink, The Fall Cup from Jen Agg’s the cocktail bar in Toronto, it totally worked. Unsweetened cranberry juice is more tart, but it can work as well.
Even the greenest cocktail newbie knows citrus must always be freshly pressed. Never, ever use lemon or lime juice out of a bottle or plastic fruit. That said, as most bars do, I will press and refrigerate my citrus in a sealed container a few hours ahead of a big cocktail night as it is unquestionably the slowest part of the process. Be sure to stir before using. If I’m mixing for just a few people, I always press my citrus at the moment.