Accounts can be queried using list-accounts | SailPoint Developer Community. This API can return a maximum of 250 accounts per API call, so you will need to use pagination to return all accounts. There are several filters supported as well. To get accounts for a particular source, use
/v3/accounts?filters=sourceId eq "2c9180858082150f0180893dbaf44201"
Conversion to a CSV file will require the use of a scripting language to convert the JSON to CSV. For a demonstration on how to perform pagination and conversion of JSON to CSV, please see this presentation, which includes a video and a github repo with the code.