I’m looking for this as well right now….. The Sleep Number sheets for our split king did not last long More on reddit.com

The Malouf Natural Supima cotton sheet set is good quality and luckily is still available in a Split King on their website. They’re sold out of most other offerings If you’re stuck on flannel, googling “split king flannel sheets” shows a lot of results but I’m not able to make a recommendation since I’ve never used them. If you don’t get a recommendation here then try to buy with a return policy so at least you don’t get stuck with anything More on reddit.com

In the past four years, we've experienced issues with two fitted sheets that ripped, despite proper care. Additionally, two grey sets developed white spots that resemble bleach stains. Our housekeeper changed the sheets either weekly or every other week. On one side, we have used three air mattresses, but the third one doesn’t hold air. The other side has its second mattress, which has developed a leak. The foam has been split since before COVID, around a year ago. Right before the lockdown, the board in the pump box failed; fortunately, the replacement part and repair were covered at no charge. Overall, the quality has been poor. However, customer service and the local technicians have been helpful in ensuring the system works by replacing items with more affordable options. More on reddit.com

You could check out sleep number. I can’t speak to the quality specifically, but they do have many options (cotton, bamboo, etc.) all that come in split top. I do know that my friend’s parents have had a full split king for years and they’ve exclusively used Sleep Number sheets for that bed and have always been satisfied. There’s actually a BOGO 50% off deal going on right now - I saw that when I googled to confirm they have split king sheets. I’d also consider checking with wherever you purchased the bed from in the first place…does the mattress company you bought the mattress from sell sheets? Good luck! More on reddit.com