Adding decorative elements into your Unity scene is just about the same as adding any other element, it's just about hunting down a model that fits your scene and positioning it so that it looks natural.
Just keep in mind that a stained glass window on the side of a cement brick wall might look out of place, so as you're searching around, keep in mind the aesthetic you're hoping to hit.
Here are a few places to look around, just keep in mind that only a percentage of all the models they offer are free, so try to check "free" in the search filter.
Once you have your standalone window/door/mantle/etc it's only a matter of arranging it adjacent to your wall, or whatever architectural element you're adorning with this detail.
Finally, as with so many other things, the devil is very much in the details here. If you're looking for a window or a door, try to get one that isn't "see through", as you'll then have to create a scene on the other side of it. Or if you're looking for a glass case, make sure you have something to put in it!