Hey, BurnedAFew, you forgot to turn off the rant mode.
[cookbook editor mode on]
Recipe writers assume -- for good or ill -- that people will follow the recipe as written if they want the dish to come out as the writer intended. They hate it when a cook says something like, "I didn't have any tomato paste, so I used tomato juice instead and it came out weak and way too runny." And they have every right to hate that. They are presenting the cook with directions how to make a very specific dish; if the cook doesn't follow those directions, it won't be the same dish. If a mashed potato recipe calls for truffle oil and you don't use it, you won't have truffled mashed potatoes; you may love or hate what you made, but you won't have what the writer wanted you to make.
But writers also have the responsibility to be as clear as possible in telling the cook what to use and how to use it, and also to make it clear to the cook what substitutions can be made without turning the dish into something completely different. Sometimes there are NO substitutions possible without completely changing the dish, either the flavor of it, or causing it to FAIL. The writer has to make sure the cook understands that.
[cookbook editor mode off]
Substitutions work only if you know what the ingredient you're replacing is like, and that what you want to use instead is very similar. If you are trying to follow a recipe and don't have something, or even know what it is, those sites that Kyle gave links for are great. And if you absolutely can't figure it out, or can't get what's needed, please, don't attempt that recipe; it will only make you mad.
And as Anneke said, if you are anywhere near a good supermarket, you can find reasonable versions of lots of "fancy" ingredients. Just remember that using, say, $3 supermarket balsamic vinegar will give you a different end product than if you had used the $40 stuff. It might taste fine to you, but it won't be what the writer had in mind.
"Coriander leaves and twigs" ????

Whose recipe is THAT? Sheesh, that's just cilantro -- but the writer should have made sure you knew that.