I haven't tried to purchase one personally, so I can't speak to it's availability in my area (and I would assume that availability will vary from area to area), but from everything I've read online (articles from around the time of its launch), you could order an iPhone 6 and have it shipped to you within 7-10 business days, while the shortest estimated delivery time for an iPhone 6+ was from the Apple online store at 3-4 weeks, with other online retailers showing everything from "no estimated delivery time frame" (Sprint) to late October (Verizon) and early November (AT&T). T-Mobile wasn't even offering iPhone 6+ devices for sale in store (save for pre-orders) at time of launch.
Even just checking att.com right now, an iPhone 6 will ship to you in 7-14 business days, while an iPhone 6+ will ship in 28-35 business days. Verizon.com states that the iPhone 6 will ship by 10/21, while the iPhone 6+ will ship by 11/14. T-Mobile just states that both will be available soon. Sprint says that they will do their best to ship the iPhone 6 within 2-4 weeks and the iPhone 6+ in 4-6 weeks. Apple online store still state 7-10 business days for the iPhone 6 and 3-4 weeks for the iPhone 6+.
So yeah, I don't have any official numbers for you, but I'm pretty sure there is a lack of availability of the iPhone 6+ and that it probably played a role of any sales shortcomings one might infer upon the device.