Here are two wondermail s with a Thunderstone as a reward one for the US/AU and one for EU
Escort a pokemon (Bulbasaur) to the first floor of Beach cave
Reward Thunderstone + ???
5#PP# 26-3J0S %=W%#
=@7@Y J#T6WM4 J-7XC
or if you on the EU one
X3S#K 75&KK5% &PK52
NXSSK C2+18KC 6T-Y8
these are very simple to do accept the job then go into beach cave and once you get onto the 1F you have done it. Simple but easy thunderstone.
Other ways of getting Thunderstone
These two you will need luck and will take longer time to get a Thunderstone
sometime from rank 3* - 9* mission
sometime from a kecleon shop inside a dungeon
Escort a pokemon (Bulbasaur) to the first floor of Beach cave
Reward Thunderstone + ???
5#PP# 26-3J0S %=W%#
=@7@Y J#T6WM4 J-7XC
or if you on the EU one
X3S#K 75&KK5% &PK52
NXSSK C2+18KC 6T-Y8
these are very simple to do accept the job then go into beach cave and once you get onto the 1F you have done it. Simple but easy thunderstone.
Other ways of getting Thunderstone
These two you will need luck and will take longer time to get a Thunderstone
sometime from rank 3* - 9* mission
sometime from a kecleon shop inside a dungeon