Before the official launch of iPhone 6 and iPhone 6 Plus, there are already quite a few cases out there in the market for the phones. I think some manufacturers bet on the leak schematics rather than waiting for Apple’s official schematics.
For iPhone 6 Plus, I found three clear cases in the market from Spigen and one from Tunewear. For Tunewear, there is only the Tunewear eggshell for iPhone 6 Plus  (S$32).
For Spigen, they are the Spigen iPhone 6 Plus Ultra Hybrid Case  (S$32), Spigen iPhone 6 Plus Capsule Case  (S$25) and Spigen iPhone 6 Plus Thin Fit Case  (S$25). All three cases can be found in Challenger stores.
The difference between the Capsule Case and Thin Fit case is the material used. Capsule Cases use Thermoplastic polyurethane (TPU) which makes it flexible, elastic, and durable. This give it a soft and comfortable grip. Thin Fit cases on the other hand is a hard case and it uses polycarbonate which makes it lightweight and durable.
For Ultra Hybrid Case, it is actually a hybrid between a case and a bumper. The case back is a hard case where as the bumper portion of it is made up of TPU that features Air Cushion Technology corners for secure drop protection.
I would recommend getting the Ultra Hybrid Case instead because the iPhone 6 Plus might be too big for some hands and it might easily slip off your hands. You will need the bumper to protect the sides of your iPhone 6 Plus when it falls.
I bought my Spigen Ultra Hybrid Case from VivoCity’s Challlenger for S$32 (after 10% member’s discount is S$28.80).