It Is Possible to Obtain "Nether Portal Blocks" In Survival
To Get Nether Portal Blocks You build a nether portal in between two Chunks and you use a specific Redstone clock (that lights, puts out, and moves a piston), a snow block and a silk touch shovel. I cant explain exactly but there are hundreds of videos explaining how!
https://www.youtube.com/watch?v=6SaEYUomw90
https://www.youtube.com/watch?v=vDiMXez5fWU
https://www.youtube.com/watch?v=YgRPmklicSk
https://www.youtube.com/watch?v=QOvaIyh7Ztg
https://www.youtube.com/watch?v=DaIPmvwMv-w
These blocks can be placed anywhere including the end, which is way to overpowered. They brake once they are updated by a solid block but they still can be used to go to the nether.
When I place one in the end it teleports me back into the same overworld stronghold, But in some rare cases in can take you to the nether! (Creative Testing). I believe it is possible to link a nether portal from the end to the nether to the overworld with precise calculations!
It also distorts the way the portal moves when you are entering it, Think of the Nausea effect but a bit worse. Once you go in with the small portal the nausea effect stays forever, even with normal portals.