What's the correct way of saying it?
"X book is available at Amazon" or "X book is available on Amazon?"
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" ... is available on ..." usually refers to websites only.
" ... is available at ..." usually refers to something being available at a particular physical location e.g "The book is available at Waterstones in Delhi."
" .. is available from ...." would be a better phrase to use as it covers both physical real locations and websites.
Assuming X is the title of the book, you could say
"X is available from Amazon and all good quality bookshops in your area".
"The book, X, is available from Amazon and all independent bookshops in your area."