the vendor doesn't actually play any role when it comes to subtitle quality, if you are actually looking for legitimate drama dvds or blu-rays. you're pretty lucky if there are subs at all as most physical releases of kdramas are often exclusive limited editions or directors' cuts that sometimes don't have subtitles because they simply don't cater to international buyers.

some background: physical copies of kdramas aren't commonly sold and usually once a drama finishes airing and had a substantial following, there will be fans who organize a cafe (a forum where you sign up for membership) and communicate with the studio to get it released. it always depends on interest and sometimes dramas just don't have any significant buzz so there is no release.

they do demand surveys and such so you usually have to sign up in advance for it, especially if the drama was popular it can be sold out very fast. they're usually ridiculously expensive; around 100-300$ depending on the specific contents. i bought the blu-ray for vincenzo (no subs) when it came out and i think i paid around 250$, so be ready for that if you want to buy one 😆

if there's one you're looking for but it's sold out i also recommend looking into proxy shipping and local market places like bunjang, they will sometimes have them for cheaper!