I just got a P0303 yesterday with a flashing CEL and engine stumbling. Previously, I've only ever gotten P0300 codes, so this was new and exciting! Problem turned out to be the Autozone Duralast spark plug wires that I picked up in a pinch late one rainy night. Would have ordered cheaper (and better) DENSO or NGK wires from rockauto.com if starting from scratch, but Autozone gives lifetime free replacements, so I just got new ones for free.I purchased brand new ignition coils from Mazda here in Australia about 3 years back, and it has "HANSH/N" and "DSC-1500" embossed near where the ignition cord plugs into it. The coils I replaced also had the same embossing, and I know the full history of the car since new, which was always serviced at a Mazda dealership. Have seen some coils that have DSC-1500 printed on them, but I'm lead to believe they're not the same/genuine Mazda.
I don't know what brand they are, and Mazda could not tell me either. It has no brand name such as Hitachi or NGK anywhere on them. The old set I replaced had done over 80'000 kilometres (just under 50'000 miles) on them, and were still fine. I replaced them as a precaution.
A quick Google of 'HANSH/N ignition coil' gives me results that also leads be to believe they're genuine Mazda.