I went to the Apple store yesterday to pick up a case for my iPhone 6 Plus. This was around 5 pm and there was a still a pretty good line outside for folks wanting to buy an iPhone 6. When I had purchased my case, the apple sales guy said they had a good quantity of iPhone 6 models, but no 6 Plus.
At this point, I think you can get your hands on an iPhone 6 pretty easily, but the 6 Plus will be awhile. I just ordered another 6 Plus from the apple store online and likely ship late October.