Is anyone aware of an API for Shiller PE ratio data?

I understand that you're looking for an API that provides Shiller PE ratio data. Unfortunately, the knowledge sources provided do not contain information about an alternative API for this specific data.

However, QuantConnect does provide a way to use custom data in your algorithms. You can import custom data from any source that provides a link to a file. This could be a potential solution if you find another source for the Shiller PE ratio data. Here is an example of how to import custom data in Python:

class CustomData(PythonData):
    def GetSource(self, config, date, isLiveMode):
        return SubscriptionDataSource("link_to_your_data_file", SubscriptionTransportMedium.RemoteFile)

    def Reader(self, config, line, date, isLiveMode):
        # parsing logic here

You can find more information about importing custom data in the QuantConnect documentation.

Please note that you would need to ensure that the data source you choose complies with their terms of service when used in this manner.