can you please tell me how i can find ornaments i don't have?

You can check Subetaloge by item type = Tree Ornaments See here

i thought they might have it so there is an option like the plushies

Sadly not. There's only 6 official collections, those show up on your profile and have the 'missing items' option. The ornaments aren't part of that.

SubetaLodge does have them as a collection, but you would have to go through them once to select the ones you do have. It might be a bit of work, but then you're good to go to check which ones are missing! :)