I have looked for translations in Thai of Stephen King, the American horror writer, but I haven't found anything.

I am a bit surprised; Thai people love ghost stories.

Yes, most of his works are, but the nature of the Thai book industry means that they get small print runs and go out of print quickly.

If you're in Bangkok, you're in luck. Until October 25, Bangkok Book Expo is going on at the Queen Sirikit Convention Centre. The semiannual fair is absolutely the best single place to find out of print and rare books, and usually at screaming good deals (probably 50-200 baht). I've seen Stephen King books at the booth the bookseller โฆษิต (website). Right now I see listed on their site book 1 and book 2 of the Dark Tower series, Dead Zone, and Nightmares & Dreamscapes. Their booth at the book fair has more titles than they list on their site, though, in my experience. If you go, just look for that name on the alphabetical list of exhibitors. If you can't, you can order through their website (a bit convoluted, but I've done it a few times), or through other booksellers.

Finding specific authors and titles is definitely hit and miss here, unless something happens to be in print. But I really enjoy the hunt. I like to collect Western literature in Thai, but the only Thai translation of King I happen to have in my collection is 'Salem's Lot.

Anyhow, just Google "สตีเฟน คิง" and you'll see there are plenty of Thai fans of Stephen King.

There are lists of Stephen King's books translated to Thai on these pages;

List 1

List 2

List 3

List 4

List 5

List 6

Thanks, the key seems to be "go out of print quickly".

I think I'll have a look at the book fair.

Here is another list. Note that none of these are available for sale, but the seller keeps a record of books they've sold. This site is particularly good if you're interested in finding out what's been translated, even if they don't actually have it in stock, because they have links on each book's page to "other books by this author" and "other books by this translator".

Then you can take the Thai titles and Google them to try to find them available from other online sellers. This is my usual method.