How do I snapshot freshness for one source only?

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

# Snapshot freshness for all Snowplow tables:
$ dbt source snapshot-freshness --select jaffle_shop
# Snapshot freshness for a particular source table:
$ dbt source snapshot-freshness --select jaffle_shop.orders
# Snapshot freshness for multiple particular source tables:
$ dbt source snapshot-freshness --select jaffle_shop.orders jaffle_shop.customers

See the source snapshot-freshness command reference for more information.