Purchasing champagne in NYC (only) stores
Just FYI, splits and half bottles aren't the same thing - a split is a quarter of a full 750 ml bottle. Pickings among splits are usually pretty slim, so you usually don't have much of a choice of brand. Half bottles are somewhat easier to come by. I haven't been in a while, but Warehouse Wines always carried a fairly good selection.
Warehouse Wines & Spirits
735 Broadway, New York, NY 10003