eBay, maybe amazon marketplace is your only resort besides buy/sell/trade here. There's a thread here that lists all the likely firmwares based on serials. Best bet is to buy a new one on eBay in which the seller doesn't know about firmwares. Only search new, sealed auctions and ask for the serial number on the box.
Otherwise I can't imagine any single retailer still having brand new N3DS on the firmware required unless gaming is truly dead in your local town and a local store still has very old stock.