You can filter on a specific source with the following query param:
GET https://{tenant}.api.identitynow.com/v3/accounts?filters=source.id eq "2c91808779ecf55c0179f11698a03905"
This will produce an array of JSON objects, as opposed to CSV. If you need the result to be in CSV formatting, you will need to process the JSON array and turn it into a CSV file. You can watch this video for more information how to convert a JSON array into a CSV file.
There is no direct replacement for exporting accounts to CSV. However, as described in the first post above, you can create a script that will query accounts and format them into a CSV response.