I know AutoZone offers a AGM Battery for the Miata, I think Majik got one not tooo long ago!
DURALAST-GOLD BATTERY for a 1996 MAZDA MX-5 MIATA
Unit Price: $79.99
Core Value*: $8.00
Part No.: 8AMU1R
Weight: 24.0 lbs.
Warranty: 8 YR
Availability: Online: Not Available
Store: Normally Stocked
Note:
SPECIAL ORDER. REQUIRES SPECIAL NON-SPILL AGM TYPE BATTERY