Only by doing the relevant rift to get them or as you said by exchanging
Opening a Rift Costs 10,000x Atlantean Coral of that Tier. Completing a Rift in Tiers 1-3 will reward you with 5,000x Coral for the next Tier Up. So, completing a Tier 1 Rift will give you your reward, and 5,000x Tier 2 Atlantean Coral.