Cooking often presents problems at the worst possible moments. The substitution question almost never arrives at a convenient time. It typically happens when the pan is already hot, only one hand is free, and you discover that an ingredient the recipe assumed you had is missing from the cupboard. Whatever resource gets consulted has to work under those exact conditions, or it will not be consulted at all.
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That reality rules out most of what is available online. A long article about substitution principles might be the right thing to read on a quiet Sunday, but it is the wrong thing when dinner is actively cooking. A forum thread is essentially a lottery, and a general search tends to return entire recipes rather than direct answers. The same constraint applies to storage questions, which is why the USDA publishes its cold food storage table as a table rather than as an essay.
What works in the moment is a direct lookup. You name the ingredient you do not have, get back what to use instead and in what proportion, and see nothing else on the screen. Resources arranged with one page per ingredient provide the format the moment demands rather than the format that reads best in leisure.
Three things make the difference between a usable answer and a frustrating one. First, the resource has to name a specific ratio rather than simply stating you can use something else, because half of all substitutions are not one for one. Second, it has to say what changes in the final dish. A cook who knows the result will be slightly sweeter can adjust accordingly, while a cook who is surprised cannot. Third, it has to distinguish structural cases, where getting it wrong ruins the dish, from flavoring cases, where it merely produces a variation.
The lookup that is genuinely worth doing before cooking rather than during is the storage check. Reaching for a substitute is often prompted by discovering that the intended ingredient has gone off, and at that point there is a second question about everything around it. The USDA FoodKeeper listings cover several hundred foods with opened and unopened windows listed separately, serving as a reliable reference for that half of the problem.
One caution about substituting under time pressure is that swapping an ingredient does not change how long the finished dish keeps. The completed meal inherits the shortest window of anything in it. For cooked food, that limit is three to four days refrigerated per the USDA leftovers and food safety guidance. A substitution made to use something up before it spoils is a good decision, but it does not buy any extra days at the other end.