The source list shows only custom audiences from the selected account. Audiences that are already lookalikes don't appear, because Meta doesn't allow creating a lookalike from another lookalike. The source must be a custom audience (engagement, website, customer list and similar). Also, Meta requires the source to have a minimum size (at least 100 people from the same country) to generate the lookalike. If the source is too small, Meta rejects the creation and the reason appears in the progress list. If the audience is new, wait for it to populate in Meta before using it as a source.