http://www.autoleads.co.uk/ would seem like the most likely, via Halfords or Motorworld.

You could also try asking here:

- MX5 owners club mail list http://autos.groups.yahoo.com/group/mx-5/
- MX5 owners club BBS http://www.mx5oc.co.uk/mx5forum/default.asp
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Remind me to change my signature to something more interesting someday