Now that the Nexus 6P and Nexus 5X are the apples of everyone's eyes, Google has (unsurprisingly) halted sales of last year's Nexus 6 on the Google Store. Alas, the Nexus 6 isn't even listed in the Google Store's Phones section anymore, having been replaced by its newer, fresher brethren.
Still, this isn't the death of the Nexus 6. Plenty of third-party retailers are still selling the smartphone, including Amazon, which has the Nexus 6 listed for $349, while Motorola is also still selling it for $499. Additionally, as part of the Nexus program, the Nexus 6 should continue to see fresh updates for the foreseeable future, so it's not a bad investment.
While the Google Store may no longer be an option, you can still grab a new Nexus 6 off of Amazon at the link below if you're interested.
Source: Google Store