Answer
The databases Journal Citation Reports (JCR) and Web of Science both provide journal impact factors.
JCR is searchable by journal name and browsable by research category. Search results include impact factors.
In Web of Science, select the journal title within search results shows the journal's impact factor.
In addition, Welch Medical Library Journals by Subject lists available journals by subject, sortable by the following journal rankings: Journal Impact Factor, Citescore, Scimago Journal & Country Rank, and Source Normalized Impact per Paper (SNIP).