I purchased the steppers from Keling. I got three KL23H276-30-8A steppers - 8 wire, 1/4" Single Shaft with flattened area on shaft.
Steppers were delivered quickly, although their packaging for shipment was not ideal and the steppers were pretty much "flopping around" inside the package. All seemed OK, and the steppers are working perfectly.
You can also secure steppers from HobbyCNC - if you think you will be needing support from Dave at HobbyCNC, then it might make excellent sense to secure as much as possible from him.
To connect the steppers to the drive screws, I used a device called a "shaft coupler with rubber spider" - it's a three-piece arrangement that provides a rubber 'spider' that isolates the motor from the drive screw - allowing for some minor mis-alignment between the drive screw and the motor shaft as well as some forgiveness in distance between the motor shaft and the drive screw. These are available from Jameco. Just search for "spider". For each stepper, I used two PN 162270 (.250" ID hub) - labeled in the image below as "couplers" and one rubber spider.
Here's the spider assembly in use. The set-screws (visible in the photo above) in the coupler were pretty small, and there is only one, so I drilled-out the existing hole an re-tapped at 6-32, and added a second set-screw at 90 degrees, just to be safe. This device is going to take a beating.