I'm training my Eelektrik up to level 55 so I can use an Eelektross against the Elite 4. Is it possible to obtain a Thunder Stone before you go to the Battle Resort after becoming the Champion? If so, where?
There is one inside New Mauville, accessible after defeating Groudon/Kyogre. Go to the center of Mauville City and speak to Wattson (who is in the centre plaza area), which will start the quest and give you access to New Mauville.
You can also receive Thunderstones by completing Follow those fleeing goals!, a Secret Super Training level.
If you fill your secret base with pals, Specifically Ninjas, Rich Boys, Pokemaniacs, Pokemon Ranger's, School kids or Street Thugs, each can unlock a skill to 'Search for Stones' - which can be used once per day to award you with any of the evolutionary stones.
Otherwise, there is a chance you will receive one as a reward from rematching Guitarist Dalton on Route 118.