I had this problem, but then I realised I'd not managed to speak to the Devon Researcher again. After you've recovered the Devon Goods, go speak to him (you'll find him at the west entrance of the Rusturf Tunnel). After that, Timer and Repeat Balls should be sold at Rustboro. You'll know if you've spoke to him at the right time, cos he'll give you a Repeat Ball.
When you finally go to capture something like the Regis, don't use your Timer Balls until later on. After every 10 rounds, the power of the Timer Ball increases effectiveness. So, you may want to use cheaper balls like Poke Balls or Great Balls, to build up that time. Ultra Balls can do the trick, but they are more expensive than even Timer Balls. Give it about 30 rounds (because only after that amount of time does the power of a Timer Ball exceed the power of an Ultra Ball) before you use them.