Try a good local shop:
http://www.autoacousticsnc.com/
I picked up an Eclipse 3403 for around $200 from them for my '94 Miata.
Check out the specs on that unit (or equivalent unit). It isn't very bling-bling, it has the essentials to output sound and not much more, but...they concentrate on making the sound output/processing circuits the best that they can for that price level (instead of the cool display that shows videos). It actually is a better deal than the head units two or three levels up the model line.
After I bought and installed mine, all my friends who were shopping for a head unit that looked at mine and others, bought the same unit. It is incredible bang for the buck and it isn't a "fast-food electronics" unit. That unit was recommended by a friend who did the competition audio installations a long time ago but has scaled back to just "something nice" for his vehicles.
Only catch: you MAY have to have them install it to get the full warranty on the unit.
Oh yeah. Audio in an M1 Miata sucks compared to a non-'vert vehicle. Don't expect to get really good output unless you osme time redoing the speakers. The only reason I even have the CD-MP3 in mine is because it is my daily driver and I hate radio, otherwise, I would say it isn't really worth it to do a "low-end bolt-in" setup (I'm told that you really need to have tweeters on the a-pillar to get better quality output).