As mentioned above, JIB is your best bet for buying HDs.
You really need to buy a HD specifically designed for RAID use. Desktop and external drives are a totally different spec and are more prone to fail early and lose data when used 24/7 in a NAS. NAS drives feature heat-resistant and anti-vibration design and provide 24/7 operation with higher speeds. They also may cost 10~15% more, but you get what you pay for.
By my estimation, 8TB drives are the sweet spot right now in terms of price vs capacity. Get yourself some 8TB Seagate IronWolf drives from JIB and you'll be set. Seagate has really efficient warranty service in Thailand, you'll usually get a replacement for a failed drive within 24 hours of returning it. And yeah, drives DO fail, so never use your NAS as a backup or as the sole location for your important files.