It might be helpful to make a list of the ranger-usable robes for your consideration:
• Guardian Robe - super-rare, but farmable. Blue-and-purple with 12ac, 5sta, 8int, 45mana, 7mr, and 7fr.
https://wiki.project1999.com/Guardian_Robe
• Robe of the Azure Sky - not sure how hardcore you are about raiding, but this wouldn't be an option for any but the most hardcore on P99.
https://wiki.project1999.com/Robe_of_the_Azure_Sky
end of list
Also, you can get a Glowing Stone Band to put on at night for Infravision; and it's actually easy to get a Helm of the Tracker, which grants Truesight (See Invis + Ultravision).
https://wiki.project1999.com/Helm_of_the_Tracker