How I can get butt plugs?
By interacting with the handsome adventurer in the forest, who brings you to the lake, and picking the right option.
What I do know is that you will need a sex interest stat at least high enough to actually have sex with the handsome adventurer.
What I don't know for sure is how many times you need to do it, or where your submissive / dominant stat needs to be sitting.
Whatever the requirement is, going to the lake with him will eventually be interrupted by a bandit. Staying quiet and going with the bandit will lead to a scene where you have anal sex with him. He gives you the butt plug afterward.
If you've found the Cum Lab, you can visit at night to unlock some interesting options. heh
If you haven't, the lady that runs it can be found in the public bath.
I know there's also a couple types of dildos, but I still haven't found them myself. I haven't done the 2nd dungeon yet so that might be why.