Hi there Ravenwolf 
It is perfectly acceptable to substitute ingredients if you need to! In fact, I highly recommend it. In my opinion, being able to substitute ingredients in a spell is a sign of growing intuition and advancing in your practice. I know Thorn Mooney has talked about it in one of her videos but I can’t remember which one.
We have even had a few discussions about substitutions here on the forum that you might find helpful.
There’s also a post dedicated to substitutions on Spells8.
The post even has some printables you can use if you want to help you find substitutions for your spells.
In my own practice, I tend to substitute as needed for different correspondences. For example, if I end up running out of Juniper which I use for purification I will substitute Rosemary which has similar properties.
Is there a specific herb or ingredient you need help substituting?