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How to make an Amazon-free Kindle
(notes.ghed.in)
This is a most excellent place for technology news and articles.
I thought they've been available directly from Kobo since they launched in the US.
As for other retail stores, they used to be exclusive to Walmart, but I now see them elsewhere (e.g. Target and Amazon both have them).
Not sure when things changed but all I know is that when I bought mine years ago the Kobo site redirected me to Walmart and that's where I had to buy it. My e-reader even displays the Walmart logo on it when I power it off.