What is a good food sub site/source?

There are many on Apple Store. (Or Play Market, if you're on Android). I personally use commercial ice makers as I work in public catering. The store I attached gives the best prices for items.

Even after culling books for a move, I kept the Food Substitutions BIble (2005) and use it often. Arranged alphabetically by ingredient you want/need to replace. Gives a range of choices when available. And some lists a & charts at the back of the book give overviews of ingredients like chiles, whole grain flours, mushrooms, etc. IACP award-winner.

https://www.amazon.com/Food-Substitutions-Bible-Ingredients-Techniques/dp/0778801195/ref=sr_1_2?keywords=food+substitutions+bible&qid=1563071010&s=gateway&sr=8-2

There's revised edition from 2010, which also gets very high recommendations.
https://www.amazon.com/Food-Substitutions-Bible-Ingredients-Techniques/dp/0778802450/ref=sr_1_1?keywords=food+substitutions+bible&qid=1563071068&s=gateway&sr=8-1

Hmm- it's not really a very unified subject- medically inclined organizations, like the American Heart Association- would be good for some types of subs, such as low sodium or low cholesterol subs. King Arthur is good for technical subs for baking, different flours, baking soda subs etc. The type of thing you're asking for, though, is purely a matter of opinion and it seems to me you would just need to find a source that you agree with, or develop your own database on your own experience- no real right or wrong about it.