You might find one at a Caravan Breakers, or Caravan Spares, but our local caravan dealer does a range of 'spare' plastic basins of two or three different shapes. Have a look at some local dealers around you, and phone around one or two caravan breakers around you (I'm not sure where you live) - there are some good ones in East Yorkshire. There is also a specialist place which will make you a basin from an existing mould - I'm not sure of the details, but think it's somewhere in Lancashire.