Is there a way to get a custom seed? As in - you can use something to get a custom seed that, and you can control what it generates - like ores and structures. Does anything like that exist? Ingame or community-made?
World generation generates a world out of a given seed which has impact at the used random variables. It's not the other way around. So you can't generate a seed out of a finished world.
For Terraria 1.3.5 exist TerraCustom which allows you to modify world generation in many ways. However it does this with changing the world generation. There will be no seed you can enter in vanilla world gen to get the same world.
If you want a seed you can use the mod Terraria Seed Project (also only for 1.3.5) there you can add a lot of world properties and search for a seed. It will generate each world and stop generating it if it does not fit your set up properties and continue with next seed. Those seeds can be entered in vanilla 1.3.5 and generate the same world (sometimes not 100% equal due to world gen bugs)
However seeds from 1.3.5 can't be used for 1.4+. World generation changed there. Those worlds only share a very few similarities.
As far as I know there is no solution fo 1.4+ out yet.
Currently I'm doing some experiments with Terraria Tweaker 2. What exactly you are looking for?