Niagara can take a week or so to pull the parts and ship them. They always show a gazillion parts in inventory, but apparently some are either not in an on-site warehouse, or are being supplied by a wholesaler, and must be acquired, re-packaged and shipped.

Ribble, Wiggle, Merlin, & etc all seem to have a more limited selection of inventory (on their internet site), but what they advertise, they generally have in stock, and it gets shipped out the next day.