I understand some things are difficult to source locally here and I might be terribly wrong here, but Las Terrenas being tourist destination and all, there is nothing locally or nearby? My mother-in-law lives in a campo and there are definitely stores for this very near... They are not Ochoa or other brand name ferreterias but locally owned businesses... if they have no such thing, they should be able to find one...
In POP, you can get one from La Sirena or any hardware store, but granted, if you need a special one, it might not fit... I hope the OP does not need to waste all that time and gas just for a toilet seat...