I have a Splunk indexer cluster (2 indexers, 1 master node), 1 search head, and multiple forwarders. Is there a way to configure source types, input ports, etc from a central web interface, or does each unit need to be configured individually?
Settings in your indexing cluster need to be pushed out from the cluster master. You'll need to put the config files you want to distribute in to the cluster master's $SPLUNK_HOME/etc/master-apps directory. There's no web UI to create the configs. You could take a test system and create the configs via the UI (say to create a new sourcetype), then copy the new configuration onto the master to distribute the changes. That's about as close as you are going to get.
Your single search head can have most configurations made through its UI, or you can push changes to it from a deployment server. But those configs would not be created in the deployment server UI. You have to put them in a specific distribution directory.
For the forwarders, you usually manage them through a deployment server, and that is a set of configurations you have to put into the deployment apps directory. So again, you could create the config in a test server ui, then copy the files that are created over to the deployment server, and it will push out changes to your forwarders.