Macau has far less places than HK! Fortress would probably be the cheapest place and with a great selection if you are looking for a wellknown brand and global warranties. There are plenty of extremely cheap Chinese knock-off brands, neither the fastest nor most rugged seals. Warranties within HK only. There are many shops around in Wai Chai (probably best selections), Mong Kok, and Shamshuipo where you can buy those, kind of electronic shopping centers.