Every site will take your order I would imagine they just can't tell you when you're getting a PS4, I would buy from a smallish retailer or PC hardware site, less people go to them but they still get regular shipments from Sony. Better chances if there's only 15 others who preordered compared to hundreds or even thousands from big online sites.
your best bet is to go to http://www.nowinstock.net/videogaming/consoles/ps4/ Just above the list of retailers, set the alarm to On and turn up your volume. When you hear it go off, one is in stock(somewhere). although its most likely a bundle. just make sure you're quick on the trigger. make an account and login on each of the sites it has listed if you don't already so its quicker. good luck!