The OpenAlex Source ID is a unique identifier that refers to venue hosting the work. This includes journals, conferences, repositories, ebook platform, book series, and other. The ID will always start with an S followed by numbers, e.g., S4121844.
You can search for Sources by going to openalex.org/sources and searching the title of the source. You can also use the filters on the right to limit your results to sources that are of a particular type, are fully open access, or are indexed in DOAJ.
Once you have selected the Source you are looking for, the URL will update to end with the Source ID, e.g., https://openalex.org/sources/s4121844 for the Journal of Applied Phycology.