I just came across these also, although I've never tried x https://www.boots.com/boots-baby-nappies-premature-size-0-24s-10299827p?srsltid=AfmBOor-h7V5C6sz04xgU2ctRxdRGuzspiUWMRelAcav3eWEgfYo9Ho0
You can email pampers and they will send you some.
The Fred and Flo preemie nappies were really good from Tesco
Sometimes you can get them at Asda via the pharmacy 🤔 however I think they give you a special number to phone if I recall it was 3yrs ago! Also I think pampers online you can purchase them..... I think you can get 5packs