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. :) :
Autoexe Type A7 Bucket Seat For Miata MX-5 2016+ | REV9
Sparco Seat EVO L QRT | Planet-Miata
Vintage Racing Seat Covers For Miata NB1/Mk2 - The Ultimate Resource for Mazda Miata Parts
PRE-ORDER Corn's Original Bucket Seat Red Stitch | website
Sparco Sprint Seat Black for Miata
Destroy or Die - Mazda MX5 Miata Side Mount Seat Brackets – Drift HQ
The Race Seat Thread - Miata Turbo Forum - Boost cars, acquire cats.
New NRG PRISMA Bucket Seat for Miata!!! - YouTube
Mazda Miata TUCKIN99 Type-RS Racing Seats
Bride Histrix Bucket Seat For Miata MX-5 | REV9
Miata Interior Restomod - Vintage Racing Bucket Seats - YouTube
REVIEW: Nakamae Type R Seats - MX-5 Miata Forum
Retro Classic Racing Bucket Seats for your Mazda Miata | KPGC10 - Miata Turbo Forum - Boost cars, acquire cats.
Bracket for Sparco seat for Mazda MX-5 Miata NA and NB OEM seat replacement | eBay
Bucket seats with door bars - Miata Turbo Forum - Boost cars, acquire cats.
Looking for some nice Corbeau seats? - Miata Turbo Forum - Boost cars, acquire cats.
Custom Mount Bracket for Sparco Sprint Seat Installation on - Etsy
Sabelt Race Duo seat mount bracket for Mazda MX5 Miata NA/NB seat rails | eBay
NA] eBay retro classic seats.. - MX-5 Miata Forum | Racing seats, Miata, Mazda mx5 miata
CarbonMiata Vintage Racing Tartan Plaid Seat Covers For Miata NA/Mk1 (Set of 2) Mazda Miata MX-5 Roadster - TopMiata