i think you can get timer balls on two island after you Complete Celio's Network Machine
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buy them+
You can buy them at the Pokemon mart at the Pokemon league. I buy my timer balls, heal balls, etc. at the Pokemon mart in canalave city
After getting the ruby and sapphire and giving them to celio go to two island and talk to the guy running the shop he will mention he has new items keep talking to him and he will soon add all the items now you can buy timer balls from him.
ummm i think you can find it on two island...
In a shop