I’m in the UK too and I’ve never come across any breeders of Saipan here on BYC.
I think your best bet would be either checking on chicken Facebook groups or placing a wanted add on a site such as Preloved.
Personally I’m iffy on buying eggs off eBay but if it’s something you are willing to do again, see if you can find the seller you got them from last time and send them a message asking if they have anymore hatching eggs.
Another suggestion would be to send an email to the Poultry Club. I haven’t seen a Saipan specific Club in the UK but the Poultry Club may be able to point you in the direction of a breeder or a similar breed you may be interested in.