For examples see Paul Goff's website
http://www.norbsa02.freeuk.com/goffybatteries.htm - he sells a complete kit in the UK but unfortunately that doesn't help you since he doesn't appear to ship batteries overseas.
You should be able to buy the empty battery case fit two Enersys Cyclon 0809-0012 6v 5.9Ah sealed cells mounted vertically and facing in opposite directions, (as he shows), lightly trimming their protruding plastic struts and/or the battery case if necessary. (not so much trimming as to weaken them). Wire in series for 12v.
I fitted a couple of vertical bars with protruding bolts outside the case to attach the wiring rather than using the spade connectors, and a fuseholder where both holder and wires were black (most have red wiring)
Beware, as the batteries are used in alarm systems and replaced on an annual or 2 year rotation, used ones are in the market and will not last as long as you would expect. (those from reputable distributors like Paul's are new)
Buy a trickle charger
These are a great solution, unless you were to fit an electric start. for which you need a higher capacity battery.