Can you get a large Urn to boil the water and make up into large jugs/pots to pour it out.
I can't imagine any rental place NOT having a coffee urn. Any church in your community will probably have a large urn they may let you borrow. Maybe your Chamber of Commerce Office? Hotel/motel -- surely you can find one! Good luck and congrats on the wedding!
It depends on how many people are attending.
Check with your church, or a local church or senior organization, offer to rent it for a nominal price if you return it in good condition!