I can't find a shop for them and some are real expensive in user shops (though I haven't had a proper look yet). I don't even know the whole list of available ornaments. This is my first year with a tree and while I can afford it I would like to RS them or something if that's how it works. (I feel like I've gotten them through vending in the past...)

Go back to Melody's Cottage, for 2k, you can buy a box of ornaments. You can do that like every 5 or 10 minutes, maybe half an hour. I don't recall.

Go to Subetalodge to look for the ornaments.

more specifically, there is no official shop for them so it's either usershops or Melody. The wait at Melody's cottage is around ten minutes. The timer for Carnival games works well to remind me to go get another box.

To be clear, if you go back to melody's cottage, you can buy a box of ornaments for 2000 sP every ten minutes.

The item she gives you looks like this:

Go to your inventory and click on it. There will be an option to "open box" and it will give you a few random ornaments (I've gotten anywhere from 2 to 5 ornaments per box!)

Happy luminaire!

Thank you all so much!! Don't think I would have found that on my own. Much appreciated!

you are welcome!

It was explained well the first time that ornaments were distributed this way, but not really any time since. Before then it was just as you thought, you could buy them in a shop. The competition for ornaments was fierce and prices skyrocketed, especially since there wasn't the fifty ornaments on your tree limit like there is now. The current method keeps the prices for most of the older ornaments low-ish.