Please note: I don't want to get into advertising individual cars - this site is enough work as it is, but I'm happy to list sites which regularly have MX5's for sale.
http://freespace.virgin.net/mx5.mike/forsale.html (11/99)
www.exchangeandmart.co.uk (12/99)
www.autotrader.co.uk (12/99)
http://www.hbrewster.freeserve.co.uk (1/00)
http://www.a1imports.co.uk/ (3/00)
http://homepage.tinet.ie/~artie/carssale-mazda.htm (3/00)
http://www.motorpoint.co.uk/ (3/00)
http://www.trade-sales.co.uk (3/00)
http://www.japanese-car-imports.co.uk/ (3/00)
Mazda dealer in Belgium http://go.to/mazda (3/00)
http://www.showroom4cars.com (4/00)
For MX5 euro-imports I can recommend www.blondeel.be They are a friendly family-run dealership in Ostend. I collected my 1.8iS from there in February. They are currently selling more cars to the UK than Belgium so are quite used to doing parallel imports. (4/00)
If you are looking for used miatae imports just visit http://www.elite-group.de (4/00)

Jap imports at:
http://homepage.tinet.ie/~artie/index.html
http://www.charlesworths.co.uk/
http://www.mixty.com/default.html
http://www.car-imports-direct.com/index.htm
http://www.japandirect.co.uk/index.asp
http://www.orient-vehicles.com/
http://www.doremotors.co.uk/
http://www.jem-vehicles.co.uk/
http://www.bestjapancar.com/
http://japcars.org/
http://www.car-imports.com/carimports.htm

As you can see, I've been looking. Can't vouch for any of them though. (4/00)
http://www.virgin.com/cars/ (4/00)
http://www.mx5centre.fsnet.co.uk (5/01)
http://www.auto-emotive.com/emotion (1/03)