I don't know where to buy them DRM free, but I can usually buy a DRM book from Amazon kindle, for example, then just download the DRM free book from a website like oceanofpdf.org
Anywhere that's not Amazon, assuming the publishers are not being assholes. Usually between the American and the British edition you can almost always find a DRM free epub version.
I've never seen that a publisher who's locking it's books would sell them different on different platforms. I just checked Andy Weir from his own website, it is all amazon / apple / google etc locked versions - or pulp-tree-and-ink.
But I'd be very happy to be wrong; have you got sites to recommend?
So, this is a website where you proof you bought the book legitimately and then, they remove the DRM software for you?
Or do i misunderstand it?
How much would they charge for this?