Java Edition:Barrier (Block)
Barrier blocks have been obtainable in a few different ways, resulting from accidents in structure files, abusing the placement of structures, and asynchronous exploits.
Obtaining
Barrier (Item)
The barrier item can be obtained via several methods and can be placed to create the block. It can be placed in any location in any dimension, allowing significantly more options than using the other two options.
Structure Population
When a structure is populated, and a tile entity is placed, if the location that the tile entity is to be placed in currently has a block with a tile entity, the game will replace that block with a barrier block to erase the data of the previous tile entity before setting the new one. By then using Update Suppression when the barrier block is placed, it halts the code and and prevents the placement of the new block, leaving a barrier block in the world. An easy setup to obtain this block is by utilizing a furnace inside an igloo. By placing a chest in place of the furnace, and then adding a comparator on the other side of the igloo, then removing the block underneath the comparator, this allows for when the barrier block to be placed to update the comparator and cause it to pop off. The comparator popping off can then be used to trigger update suppression and leave the barrier block.
This must be done in 17w46 (1.13 snapshot) or before, as updates are not produced from world generation in 17w47a and later.
Kerbaras' explanation (Archived Video)
EarthComputer's first reveal of the barrier block from this method
Bastion Remnant
A barrier block was accidentally left in the structure files for the bastion remnant when they were first introduced in 20w16a.[1] This was fixed the very next snapshot. The specific location of the barrier is in the lava pool at the end in the bastion/treasure/ramparts/lava_basin_main structure. 1 of 4 bastions will be the treasure variant, which contains the structure. Only one lava basin main can be found per bastion remnant, so only 1 barrier can generate per bastion remnant.
Barrier in lava pool at a bastion remnant
Lava basin main section of the bastion remnant
Word Tearing and Any Falling Block via Asynchronous Threads
From 14w32a (1.8 snapshot) to 17w46a (1.13 snapshot), asynchronous block updates allow for block palette corruption, and this corruption, when combined with the registry palette type, can allow for the creation of any block that can exist, such as barriers. Asynchronous block updates also allow for usage of the "generic method" to create a falling block entity of any existing block in your world.
These falling blocks may also be used to obtain their respective item, if it exists in item form, such as end portal frames. Blocks such as end gateways cannot be obtained as items with this method, as they have no corresponding item. Falling blocks also allow for the placement of both blocks that cannot normally be placed, either due to operator restrictions (e.g. command blocks) or not having an item form (e.g. end gateways).
10 Years of Minecraft
Barriers can be found in many places on the 10 Years of Minecraft map.