Some of my links are no longer working, so your mileage may vary and you may have to just search. Here are a couple that are working:
PofoWIKI (has 3rd party and DIY section - german, link does translation):
https://www-pofowiki-de.translate.goog/doku.php?_x_tr_sch=http&_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en
RSKey Portfolio Page (has bootstrap program to xfer a usable terminal, or used to):
https://www.rskey.org/pfolio
Software (U of Mich archive):
https://websites.umich.edu/~archive/atari/Portfolio/
And if your interested, I did a podcast called Podfolio, Atari Portfolio podcast which should give you plenty of information about the device and its accessories:
https://inverseatascii.info/episodes-podfolio/
I think it was this link that had the backlight kits:
http://www.palmzip.de (german)
HTH
Wade