There don't seem to be a huge number of options out there, but we do list a few sources that indicate their anti-RORgamma(t) mAbs are suitable for FC. Here are our listings: https://www.linscottsdirectory.com/antibodies?q=RORgamma
You can filter for FC, and follow "more info" to each supplier's data pages.
This eBio clone is widely referenced and works nicely in flow cytometry: https://app.benchsci.com/product/Thermo%20Fisher%20Scientific/12-6988-82/figures