9 speed rings will work fine, whoever has told you otherwise doesn’t really know what they’re talking about.
Running 24/36/bash here on one of my bikes with a 10 speed setup, and it’s all good. There are others on here doing similarly, and I spent a while researching the net and asking on here before hand too to see if it would work. It does!
EDIT: I’m using 9spd Raceface rings as that’s what I had lying around in my toolbox spare, but I’d just buy a cheapo steel Deore 36T ring if I had to buy one. They last longer than pretty much everything else, and the big thing you notice when you go to dual, or even worse single, ring setups is that the chainrings wear faster than with a triple up front.