They aren't incredibly hard to find, especially with the internet.

Another interesting alernative would be a 6/12 volt battery - which allows for a 'partial' conversion of a car. Convert the charging system nd starter to 12 volts, but tap the rest of the car's power supply of the 6 volt terminal of the 3 terminal battery.

http://www.antiqueautobattery.com/prod011.htm

(They also have 8 volt batteries and other odd batteries that put out 6 volts normally but switch to 12 for cranking)

I used to have a 6-volt VW Beetle and that was a REAL P.I.T.A. to start using the key. I always had to park it on a hill. You had one good chance at the starter getting the motor to fire up, then a couple of long shots, and that was it. New starter, new battery, new cables, didn't help much, or for long. But the 8-volt battery (found at a tractor supply store) fixed it really well. No conversion necessary other than tweaking the adjuster tab on the regulator, nice bright light bulbs (for once in their lives) that didn't burn out any faster, and a starter that actually acted like it gave a hoot!
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I'm JohnMc, and I approved this message.