as title says

You can't. Closest would be Nord Hero Sword, which involves spoilers.

Honed Ancient Swords and Axes, as well as "Supple" ancient Nord Bows can only be found as loot,
About one in 20 drops, I'd guess.

they are usually carried by high level draugr, like Wights or Deathlords.

As already stated, only in drops. I got my first one from the Galdur quests. One of the idiot evil sons had it as his secondary weapon. Mine was actually enchanted with 30 Fire Damage, which makes it a pretty nice hitting weapon after you take it to a grindstone and make it legendary.

Deathlords seem to have them about at about 50% drop chance after level 25-30 from what I have seen anyway.

I found I could not upgrade them at least not the bow it did not show up on the list, has to check again.

@ Zaria-
I think it's because I am using a smithing mod. Not sure if you can upgrade in regular vanilla smithing.
I am using this mod "Lost Art of the Blacksmith". It lets you upgrade to legendary stats all the armor and weapons that you normally can not do that with.

The "Supple" bow cannot be improved without a mod (probably an oversight), though the ordinary Ancient Nord Bow can be.
Both normal and "honed" swords and axes can be improved at wheels and are affected by the steel perk.

I think that you can make the weapons at the skyforge under Daedric.