How/Where do you get a solar tablet?

Accroding to the Wiki they

[...] can be found in Lihzahrd Chests in the Jungle Temple.

and can also be made from Fragments which can be found from:

[...] in Lihzahrd Chests, and are dropped by Lihzahrds and Flying Snakes in the Jungle Temple.

and used at a Mythril or Orichalum anvil (the only two hardmode anvils) (Thanks @Aequitas)

Solar Tablets can be found in the Jungle Temple, located deep in the Underground Jungle. They can also be crafted with Solar Tablet Fragments, which are dropped plentifully from Lihzahrds and Flying Snakes.

Kill plantera, that will give you a temple key. Go to the jungle temple and open the door with the temple key. Go inside and you will find solar tablets and solar tablet fragments.

Buy poop from the traveling merchant for 1 platinum each. You will need 6 poop to make solar tablet fragments - 2 poop for each fragment. Use 3 fragments to make a solar tablet.