purveyor (to the Imperial Household, etc.)
company (or restaurant, place, etc.) that caters to or is popular with a specific clientele
このお菓子は宮内庁御用達だ。
This confectionery is a purveyor to the Imperial Household.
雑誌で芸能人御用達の店が紹介された。
A restaurant favored by celebrities was featured in a magazine.
このブランドは彼にとっての御用達となった。
This brand became his go-to.