Update on the OMP Champ-R bucket seat, it wil fit in a NA Miata with some custom brackets. But be careful if you're around or above 185cm you'll won't have enough legroom. :) :
Destroy or Die - Mazda MX5 Miata Side Mount Seat Brackets – Drift HQ
Ultimate Miata Seats Guide | Drifted.com
NB Miata (99-05) Interior Seats | Planet-Miata
Ultimate Miata Seats Guide | Drifted.com
Mazdaspeed NR-A Racing Seat For Miata MX5 | REV9
TUCKIN99 Type-RS Racing Seats For Miata MX5 | REV9
I Fixed My Track-Prepped Mazda Miata's Biggest Flaw
Fixed Position Racing Bucket Seats w/ Sliders For Mazda Miata MX5 NA NB RX7 RX8 | eBay
Seat options for tall people...... in a miata| Builds and Project Cars forum
NA] eBay retro classic seats.. - MX-5 Miata Forum | Racing seats, Miata, Dune buggy