I don't have a particular place to recommend but would definitely try Northern Ireland as I know prices there are much more reasonable. And if you're buying in bulk, use those negotiation skills you honed when dealing with the reception venue and try to negotiate a deal
Also, I was at a party with friends the other week and we had 4 bottles of different wine from Lidl. Lovely stuff and cheaper than normal. Depends on if you want a known label or not.
Careful on corkage, our place is charging us €8 for wine and €10 for prosecco/champagne which does add up....but still cheaper sourcing your own.