Get a single source

It's easy to get a source from from the API with: /sources/<entity_id>. Here's an example:
That will return an Source object, describing everything OpenAlex knows about the source with that ID:
{
"id": "https://openalex.org/S137773608",
"issn_l": "0028-0836",
"issn": [
"1476-4687",
"0028-0836"
],
"display_name": "Nature",
// other fields removed for brevity
}
You can make up to 50 of these queries at once by requesting a list of entities and filtering on IDs using OR syntax.
Sources are also available via an alias: /journals

External IDs

You can look up journals using external IDs such as an ISSN:
Available external IDs for sources are:
External ID
URN
ISSN
issn
Fatcat
fatcat
Microsoft Academic Graph (MAG)
mag
Wikidata
wikidata

Select fields

You can use select to limit the fields that are returned in a source object. More details are here.