MELCHIORS is a new library of well calibrated very high resolution spectra which looks like it could be very useful for flux calibrating (response correcting) spectra. (Thanks to Christian Buil on the SolEx forum for bringing this to the attention of the spectroscopic community)
https://www.royer.se/melchiors.html
The fits files are not easy to read but you can quickly view the spectra at the website and I expect there will be tools produced to read the fits files (Christian has added one to SpecInti)
Cheers
Robin