...What 2nd hand ones fit?
There's an often-quoted list on miataturbo.net:
Year Make/Model Engine Size/Color Part #
87-88 Toyota MR2 4AGE NA 213cc – Beige Top #23250-16080
89-91 B2220 Truck 224cc - Yellow Top? #23250-74040?
99-00 Mazda Miata 240cc - Thin Red Body #195500-4430
01-06 Mazda Miata 240cc - Thin Lt. Prple #195500-4060
Toyota 4AGE 250cc – Green Top
Toyota 4AGE 250cc – Violet Top
94-97 Mazda Miata 265cc - Tan Top #195500-2180
Toyota 3SGE 295cc – Green Top
89-92 Toyota Supra 7MGE NA 305cc – LT Green Top #23250-70080
93-95 Toyota Supra 3.0L 312cc - Maroon Top #23250-46030
Toyota 3SGE 315cc – Pink Top
90-92 MX6/626/Probe Turbo 326cc - Gray Top #195500-2150
89-91 B2600 Truck 326cc - Gray Top #195500-2150
Mazda 323 GTX TURBO 360cc - Black Top #195500-2130
Celica/MR2 3SGE NA 370cc - Green Top #23250-74160
Mazda RX8 420cc - Yellow Body #195500-4450
89-92 Mazda RX-7 NA 440cc - Blue Top #195500-5740
89-91 RX-7 NA 460cc - Red Top #195500-2010
89-91 RX-7 TURBO 550cc - purple top #195500-2020
(You'll see that the tan 1.8 MX-5 injectors are listed there as 265cc/min instead of 230. I don't know why.)
All of the above are high impedance injectors with the same Denso connector as stock, so they're electrically and mechanically compatible with MX-5s, but you need to be aware that the stock ECU won't be able to control injectors much larger than the stock ones unaided. For example I believe the 1.6 ECU can cope with swapping its stock blue-top 205cc injectors for 230cc tan tops from a 1.8, but any larger size needs some supplementary electronics (or a replacement ECU) to enable it to cope.
If you did go down the route of changing the ECU then you would also be able to consider using faster-opening low impedance injectors instead. (There's a similar list of those types. They use an almost identical Denso connector but with a tab that needs to be filed down to make them plug and play.)
Whn it comes to new injectors, there's a model (EV14, I think it's called) which quite a number of people have tried and seem to like. There's some kind of issue around making sure you get the right spacers with it, as it's physically shorter than the Denso pattern. Don't know the exact details though.