There was a great website called gourmetsleuth.com . I've recommended it on MN in the past.
Amongst other things, it was very useful for suggesting substitutes for ingredients that were hard to find, & also how to make things such as buttermilk at home.
That site has disappeared. I've found others that give limited help on common ingredients, but I'm looking for a more comprehensive site.
e.g. My old jar of sumac smells of nothing; I can't find pul biber - apart from online at exorbitant cost.
Anyone know of a good site, please ?