When can you find the oases

[Tapu Fufu]

I'm with Taka and while I got Return and saw the murky water where Mirage Tower is supposed to appear, I can't seem to find even a single oasis. It's odd, because I looked at a walkthrough, and it seems the oases have already disappeared for me. 

[Dreamy]

There should always be one in existance at any one time. when you make one dissapear with the shard, a different one appears and mirage tower has a chance of appearing

[Alistair]

^ what DreamblitzX said + I think you have to find it 3 times before Mirage Tower can appear

[Tapu Fufu]

Nevermind, I found one. I think they're just more difficult to find when you're with Taka. I don't remember them being this difficult to find in my other playthrough. I didn't know only one could appear at a time, though. That's good to know.