I've got a box of used Ford electric horns in my shed, all from Mk1 XR4i or Mk2 XR4x4 - they look the same.

They have two different tones though, so you will want the deeper one if yours is a bit "camp" - from memory, I think they have a code moulded on the side that would positively identify which one of yours is working.

Don't know what they cost to post but I'll box one and send it for £8.