mx5cartalk.com

I believe any single DIN size stereo will fit, what features you get are up to the depth of your pockets.
For wiring, see if you can find a wiring harness Mazda-to-ISO and another from ISO-to-Head Unit, much easier and neater than hacking into the cars wiring

Be cautious letting someone install it for you as there have been a few horror stories.

http://www.mx5parts.co.uk/product_info. ... cts_id/617
then all you need is a single din headunit

You can get adaptor harnesses to suit the OEM wiring loom so most new head units are "plug and play" some "plug in" wiring looms still expect you to connect a couple of wires but it's still easier than doing the whole thing and quickly reversible.