Whether your battery holder can be replaced with a larger tray depends on the particular model, although I thought that most Powercaddy models will take a 36 hour battery. Just make sure that any battery ordered will fit the tray dimensions.
I have always just use Google, eBay or Amazon to order new batteries (normally your local golf pro will charge the full retail price, whereas internet prices are a lot cheaper). Try someone like:
http://golftrolleybatteries.co.uk/batteries-for-powakaddy-trolleys
for a price comparison and remember that Powercaddy trolleys usually use a 'T-Bar' connector.