Generally, craft stores sell only a handful of different threadcounts of a given type of fabric, usually very low threadcount, but in dozens of different patterns, because customers are buying based on patterns, not fabric density. Also, higher threadcount fabrics are more expensive and thus less in demand, so generally need to be purchased special order. Alternatively, you can buy directly from a clothing/fabric wholesaler (though the wholesaler may ask for a reseller's license first).
No, it is not; that's what parent was getting at.
"The amount of thread that can fit into a square inch of fabric is limited, suggesting that bedding beyond 400 count is likely a marketing strategy." https://en.wikipedia.org/wiki/Units_of_textile_measurement#T...