A neat website to help you find alternatives is YarnSub.
You can simply type in the name of the yarn you are looking for and it will give you multiple comparables, even telling you how close of a match the yarn is!
As an example, a project I have just loaded into my queue uses Caron One Pound yarn. This is awesome as I have two of those skeins in my stash. But since I like to be different than everyone else, I went searching for varying colors and found the colors I want are not in stock anywhere near me.
So I went to YarnSub and typed it in and found Lion Brand Heartland and Red Heart with Love are good substitutes. I can search those brands online and see if they have the color schemes I'm looking for in stock near me.
Very simple and easy and I can make sure I'm going to get something that will work with the other yarns in my project. Win win!
Do you use YarnSub? How has it helped you on your crafting journey?