When you finished up with Nelacar, he told you if you return to him, he will give you keys in exchange for.... well you know what he wants (if you've played far enough). The Curious Clothing quest which remains active hints about this to remind you.
Nelacar will randomly give either a tiny or regular key. For Exotic you need to agree to calling his penis tiny, otherwise don't call him tiny and you get Basic. That covers the 4 possible keys.
But more about keys.... A key can be:
a Toys Key - one of 4 that are part of Toys. Basic, Basic Tiny, Exotic, and Exotic Tiny, are the 4
or a Custom Key - a key that is part of a content mod, so often part of a quest
For Toys Keys.... if install you the ToyFul Temptations mod (Toys Family of Mods for link), it will put Toys Keys in the world to be found. That's the Default way to get Toys Keys.
Note: If you are adding ToyFul Temptations mod mid-playthrough, be sure to follow the golden rule (you can do so by placing it at the end of your load order).