How do I snapshot freshness for one source only?

Use the --select flag to snapshot freshness for specific sources. Eg:

# Snapshot freshness for all Jaffle Shop tables:
$ dbt source freshness --select source:jaffle_shop

# Snapshot freshness for a particular source <Term id="table" />:
$ dbt source freshness --select source:jaffle_shop.orders

# Snapshot freshness for multiple particular source tables:
$ dbt source freshness --select source:jaffle_shop.orders source:jaffle_shop.customers

See the source freshness command reference for more information.