Things can be worse for more independent or self-published works where the author doesn’t know how these things work. These may often have no ISBN at all regardless of the format.
The least “indie” thing one could do would be to use the Amazon Standard Identification Number which is a number assigned by Amazon. ASINs are easy to find on Goodreads solely because they’re owned by Amazon. In many cases, there are far more editions on Goodreads than actually exist because of the lack of use of ISBNs and de-duplication of editions which they import from a variety of data sources, including Amazon itself.
To my knowledge, the only true way to find the “correct” ISBN is to copy it directly from the book/source itself.