The Google Store has been the sole source for new Nexus phones thus far, but now the Nexus 5X is expanding. This device is now available officially on Amazon and B&H Photo. All the colors and capacities are listed, but there's no way to get Google's Nexus Protect add-on if you buy from these outlets.
Amazon and B&H are both listing the device for the standard $379/429 price points. So depending on where you live, the final price for the Nexus 5X could be much lower than the Google store, which charges tax and shipping. It looks like everything is available from both retailers for shipping today, with the exception of the 16GB ice blue phone. Also, this color is being called "mint" now, I guess.
This is the same device you can get from the Google Store. The only notable difference is that the $69 Nexus Protect plan isn't available. That means you're at the mercy of LG's warranty service if something goes wrong.