My guess is that it's similar to Jeans, in that older clothes have a different feel and texture to them.
I mean, there is no way a piece of clothes created say 20-50 years ago would be of the same quality as a brand new one.
There is bound to be some degradation in quality of the cloth, which in a way should fit a character based on a Fallen Angel.
This is Japan. There are kimono makers that have been making kimonos for 50, 60, 75+ years without changing.
Even the jeans you cite, no, there's not really much of a difference at all between jeans from today and 20-50 years ago. There are ink makers in Japan that make ink for calligraphy
in the same process that is literally unaltered for hundreds of years
From some quick research (and holy shit is it not clear as to what "antique kimono" actually means and is usually just a catch-all marketing word), but "antique kimono" just simply refers to 100% silk kimonos done in traditional style. Sometime around the 1960s and 1970s silk-blends started being used because 100% silk was getting too expensive to use. After that cotton/poly blends also started being used.
So all you're talking about is getting a traditional kimono made in authentic 100% silk. Which can easily be found for less than $200. And that's import. It'd be even cheaper for Gojou. Still somewhat expensive for a high school student to spend just on fabric, but not outrageous and still within the same ballpark that they've previously spent on cosplay.
Gojou is struggling with it because it's not something he's familiar with because he's 1.) Young and 2.) Not a dress/textile maker. He's not searching for the fabric version of the Ark of the Covenant here, it's just the equivalent of asking your average teenager to name something from before they were born... like a decent band or a movie that isn't a Marvel/Disney movie from the last several years.