A new Splunk Firepower solution is now available if you are using Firepower version 6.x. You can download the new eStreamer eNcore for Splunk and the separately installable dashboard from the two links below:
It is free to use and well documented but if you would like to purchase a TAC Support service so that you can obtain installation and configuration assistance and troubleshooting you can order the software from Cisco (support obligatory with this purchase). The Product Identifier is: FP-SPLUNK-SW-K9.
Regardless of whether you take up the support option or not, updated versions will be made available to all free of charge and posted on Splunkbase as well as Cisco Downloads.