Wilwood 1" master cylinder
 

After the https://supermiata.com/miata-wilwood-master-cylinder.aspx

In the UK.
Also, have a http://www.compbrake.com/product/mazda-mx5-brake-bias-servo-replacement-pedal-box-kit-hydraulic-dual-cylinder-kit-a/ for sale, or exchange, basically brand new, used once