SOLUTION TO YOUR PROBLEM
UPS & Telecom AGM Batteries | BatteryStuff.com
Visit the link and match up the dimensions of the batteries to fit those shown on the page. I really like the AGM's because they hold their charge for months when sitting unused. I use this website to build small scale battery banks for my battery charger research.
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Here are their 36VDC battery chargers. This one:
Japlar 36V 4 amp looks like the cheapest.
Good luck!