No hardware store I have ever been to (and I spend a LOT of time in hardware stores) has proper JIS metric. Neither does AAA (BTDT). I either special order from Fastenal or buy from dealer.
Short primer: Two main kinds of metric (1) DIN/ISO is what you find in hardware stores. For example an M8 x 1.25 fastener will have a 13mm head. (2) JIS is Japanese Industrial Standard. That same M8 x 1.25 fastener will have the proper 12mm head. Makes a big difference in certain applications, such as where clearance is an issue. For example carb mounting nuts and exhaust flanges. Smaller difference is not having to carry 13mm or 15mm wrenches.
Dealer pricing on fasteners is surprisingly competitive with aftermarket. When I buy from fastenal I buy 100 lots for good price breaks. Note that there are a fair number of special sized fasteners which are really best gotten from dealer, examples include exhaust and engine/tranny related fasteners.