I’ve also been wanting to run something with the King in Yellow. Two Chaosium books I know of focused specifically on Hastur are the aforementioned Ripples from Carcosa as well as Tatters of the King. The latter is a 6th edition book so you’ll have to do some conversions. smithh65 also mentioned “Tell Me, Have You Seen the Yellow Sign?” The updated version in Tales from the Crescent City isn’t on DriveThruRPG but the original version from The Great Old Ones, a series of modules for 4th edition, is. However, if you just want to get a copy of Tales through DTRPG for a digital copy to make handouts, you can always go to Golden Goblin Press’s website. They have digital and hard copies. Another book on DTRPG is John Wick’s Curse of the Yellow Sign.
Personally, I’ve been toying with the idea of retooling “A Night at the Opera” from Tales from the Sleepless City into a Hastur story, swapping the opera house for a theatre, switching “Massa di Requiem per Shuggay” for “The King in Yellow”, and Azathoth for Hastur. Unfortunately that collection isn’t on DTRPG either.
Golden Goblin Press’ Tales of the Crescent City
https://www.goldengoblinpress.com/store/#!/Tales-of-the-Crescent-City-Adventures-in-Jazz-Era-New-Orleans-Digital-Download/p/37692597/category=6641143