Lost a wheel mudguard (possible called a spat)

Hi everyone.  I've got a Crystal Sprint 450TF which I love to bits.

At the weekend, I was towing her.  When I arrived at my destination I found that the plastic trim which makes the wheel arch look nice has come off.  Of course, I didn't see where it happened, so the part has gone never to be seen again. (I've already been out hunting for it, but couldn't see it)

So, I need replace the part.  Can anyone recommend somewhere that I can go to come a replacement part?

I'm planning on taking photos and getting measurements after work.  Hopefully. it will be possible to get something that will make her look good again.