At this moment I think non Tesla owners are going to be at the mercy of Electrify America for DC fast charging. Hopefully by the time Rivian hits the road they will have placed a fair number of super chargers around the country.
https://www.electrifyamerica.com/

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Charging stations are slowly starting to appear since this dated map was posted. I'm some what surprised with lack of interest in members being that Electrify America is critical to the advancement of Rivian and other non Tesla EV's.
Here’s a map of Electrify America planned buildout from 3 months ago. News since then is that Walmart has made a deal to install these in many of it’s supercenters, and some may be on line now.
The hook is that, although they all will be capable of fast charging, expect only a few to be enabled for such until the demand ramps up. Also the EA plan explicitly says something like 40% of their budget will be spent on California alone. BUT, VW seems likely to add more funding when they launch their own fleet of BEVs to the US.