I'd recommend registering "wants" on Advanced Book Exchange; it's free, and you automatically get e-mailed when books show up on ABE (sometimes other people beat you to them--e.g. I've been beaten to a $20 DEVIL KINSMERE and a $17 HANGMAN'S HANDYMAN, books that usually see for hundreds of dollars--but I've also got some bargains). Also, don't restrict yourself to UK booksellers; I've purchased books on ABE from around the globe--Australia, Switzerland, South Africa, you name it. If you're patient enough to request surface mail, the shipping costs usually aren't too bad.
OBELISTS AT SEA was published by Penguin Books, which may be your best bet. But the only copy on
www.bookfinder.com is about $50, so you may want to register a want for that also. (I'm visiting the UK in May and if I see a cheap copy of this, I'll let you know.)
As far as I can tell from the WorldCat database, the only books by King with the word "Obelists" in the title are OBELISTA AT SEA, OBELISTS EN ROUTE, and OBELISTS FLY HIGH. But there may be others in the series without "Obelsits" in the title.
I don't know what Obelists are, either. I had thought it might have something to do with oboes or obelisks, but the word isn't even in the Oxford English Dictionary. (The verb "obelize" means to mark a word with an obelus, i.e. "to condemn as spurious or corrupt"; does this make sense? Do the Obelists expose fakes? I've heard of these books but never read them.)
Good luck,
Arthur