If you need the BSDF, you may need to get it measured.

There are however a few curves in the literature (from a quick google search) with the reflectance as function of incidence angle and wavelength

Fig. 19 of:

https://wp.optics.arizona.edu/optomech/wp-content/uploads/sites/53/2016/10/persky-1999.pdf

Fig. 4 from:

https://www.researchgate.net/publication/252807685_The_Geosynchronous_Imaging_Fourier_Transform_Spectrometer_GIFTS_on-board_blackbody_calibration_system

Other information (reflectance and various angles)
https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1183&context=star

Thank you for your detailed answer, but I don’t have the equipment to measure this data.

The Zemax format for .bsdf data is a bit different—it needs to be directly obtained from a measurement machine. Since I can’t generate sufficient data with graphs alone, I wanted to ask if there is anyone who has already measured this data in a format suitable for Zemax.