They are typically available at the front desk or in the lab area (specifically Lab B) where you originally received them during the story. Cost: $1,000 Poké Dollars per ball.
While buying them at Aether Paradise is the most straightforward method in the "Ultra" versions, there are other ways to obtain them: where to buy beast balls ultra sun and moon
You must defeat Necrozma at Megalo Tower and rescue Lusamine and Guzma to unlock the ability to buy them. Other Ways to Get Beast Balls They are typically available at the front desk