If you have a non-standard toilet, you need to find the manufacturer and approach them for the correct size seat.
Most toilets have a logo between the seat posts. It might only state the model, but some have the full name. If you can’t see any logo, lift the toilet tank lid and check underneath. It should be printed there.
You should be able to search the manufacturer’s website to source the replacement seat. It could also be a European toilet seat, which might explain why it is non-standard in size. While the designs may be similar to US seats, European styles also include D-shaped seats.
Sourcing a replacement D-shaped seat is pretty easy because you can order it online and get it shipped overseas to the US.