There are tree houses in the game - I think it is "into the future's houses. They have different Postbox and Trash can's. But I just can't find it.

The postbox should be the one from the Store World Lunar Lakes, if you own the world you can find it in the buydebug category.

No, this is not part of a Store World, but came with the last Expansion Packs – “Into the Future”
When your game, Sims 3, is open – on top left are icons for Egypt – University exe and “OASIS LANDING”
There are these Future houses, with that postbox and Trashcan. Even if you do the cheats – Testingcheatsenabled true + Buydebug on – Still I can’t find that specific Postbox or Trashcan. That’s why I asked if there is someone who known how to get it for any other house you want to build?

If you are in debug buy mode, don't filter for ITF. Expansion mailboxes show up as base game items. When filtering for base game items, it showed up pretty much near the top for me.

As far as the bin goes, I didn't even know there was a special one for ITF because I've never been able to find it. I looked it up in s3oc. It was missing the catalog flags. Here is an override that will allow it to be seen in debug buy mode. Again, filter for base game items.

You're right, the Postbox from Lunar Lakes looks different.
I thought it must be the right one, because I deactivated ITF since years and used the Postbox from ITF a few weeks ago