Where can I find the Scanner in the Sea Mauville in ORAS?

Question

I`m looking for it to eventually get Lugia...

Answer 1

Firstly you need to complete the Delta Episode. Then, follow these instructions (source):

After the Delta Episode, go to Sea Mauville and get the Scanner in the basement. Take the scanner to Captain Stern in Slateport City and he will give you either the Clear Bell (Omega Ruby) or the Tidal Bell (Alpha Sapphire).

The Scanner itself is found in the basement near the stairs. This video will help.

Comments

It is not necessary to complete the Delta Episode.  I found the scanner in that location without even defeating the Elite Four.  The only requirement I am aware of was getting past Kyogre or Groudon.