You can do one of two three things.

Buy a B6AC-18-520 which is a Panasonic designation S46A24L(S) Vented battery, for the approximately $300.00 and spend five minutes fitting the battery. Start your car and drive away.

OR

Buy a Delkor 22NF-330D vented battery from Independent Batteries 8260 6111 their model number is MF39-40M (they will know what one is), do some minor modifications to your MX-5's battery clamp, add some tubing and cheap parts and 30 minutes later be starting your car and driving away.

Why would you do the second I hear you ask?? The total project cost me less than $135.00, and I still have leftovers to do more!

Parts required:

MF39-40M Battery cost me $124.00
Poly Tube (dripper or sprinkler) inside diameter of 4mm 1 meter cost me $2.00
4mm barbed "Tee" pack of 10 cost me $4.00

Vice, hammer and hack saw, the hammer is to beat out the drivers side guard to fit the battery, the saw is to cut a section of the battery clamp off and the vice is to give you a helping hand.OK, I'm joking about the hammer!

The new battery (228w x 185h x 135d) is slightly larger than the OE (Original Equipment) battery (235w x 160h x 125d), so I had to remove the drip tray, I am looking for a replacement as I type this, so watch this space for more details, the new battery is semi- sealed and maintenance free as is the OE one. The other modification to fit this battery is the lengthening of the retaining bolt that clamps the battery, to save cost I straightened the existing one and then re bent it to make it longer, it is only just too short, a hammer and vice help here

The new battery has a vent hole right where the battery clamp rests, so for this to be connected to your existing tubing you need to cut away a section to make way for the tubing. You can see the section I cut away in the photo below.

The drip tube will need to be chamfered slightly to help push it into the vent hole on the battery, the barbed "Tee" is pushed into the tube very easily, and then the two existing hoses are connected to the "Tee", then all you need to do is put the clamp on and away you go.

I cut a section about 15mm out of the battery clamp, this has not weakened the clamp that much and the clamp still holds the battery very firmly and securely. The section that was cut could be smaller or better still a hole could be drilled, but it would involve a little more precision, and I was a little rushed, finally I gave the clamp a good coat of paint and let it dry.