probably any displayport fiber optic extension or cable that advertises ”8k” ”dp1.4” and/or something like ”vr compatible”. look/search the reviews and questions for ”index” or ”valve” or ”vive pro”.
if you get one with 2 male ends, you will need an f-f passive adapter, like
or
one dp fiber optic cable i'd check out is this guy. i can't guarantee it'll work, but if i was putting money down on a bet, i'd bet on this cable.
or you can search for ”displayport fiber optic 8k 1.4” and nose around the reviews to see if anyone mentioned the valve index.
or you could search this subreddit for ”fiber” and your will likely find a few posts or more recommend fiber extensions that work for the person making the recommendation.