In the strategy guide it mentions that it can also be found and not just made. If so is it just a random drop from containers?

Yes. I haven't found any, but I know some people who have.

I found in Autumnwatch.

edit: it was an enchanted plate helm.

I found a Dragonplate helm on a shelf in a tomb, but finds like that seem to be very rare.

its an INCREDIBLY rare drop
i wouldnt count on collecting a full set by dungeon delving alone
ive only found the helm after 100 hours of play with a level 81 char, so its loot appearance rate must be microscopic

A little over 218 hours of gameplay, level 65. I've yet to see any as loot. If I want it, I can make it though.

I've never found any. Maybe because I'm not that far in the main storyline. Though it seems likely that others would discover how to make dragon armor soon. I mean, it doesn't take much. Once dragons become rampant in Skyrim then anyone could kill one and just use its bones.

So it's not worth investing in smithing perks? As eventually, everything craftable can be found and you can use enchantments and potions to boost your skill level high enough to improve gear to Legendary status.

Can Ancient Nord Armour be found too or can it only be crafted at .......... (no spoiler)?

Well, you will get those armour sets much,much faster by using smith perks and making them yourself and you will also be able to upgrade them properly.

It is all down to how you want to play, If you prefer to find your loot so you will have something long term to work for then good for you, but it will be a very long and frustrating process where Dragon armour is concerned.

I believe I have seen Ancient Nord armour drop in cryps etc. but I ignored it and figured it was in the same league as Ancient Nord weapons which all Draugr drop.

To be honest I'm not bothered about the Dragon Armour, I'm currently level 25 and have 3 pieces of Legendary Steel (body, hands & feet) and an exquisite Iron Helm. The Ancient Nord Armour is the set that I really want.

I've found it once in all the hours I've played. It was one of the light armor chestpieces. As I recall it was in a chest near a word wall shortly after beating a Deathlord or whatever the next step up from him is.

Found a Dragonbone Armor and a Dragonscale Shield.
That's about all I've found.
But I've crafted about 4 Dragonbone Armors. They're just too easy to make.
Edit: Daedric Armor on the other hand is pretty hard to make.

Just checked the wiki and Ancient Nord Armour can indeed be found, there is one particular dungeon where you will find the whole set.
I wont say where or post links and spoil your game though so good luck with it and have fun.

just make some you get 1 billion dragon bones

Found a heavy armor in a random chest (I use light obv), but it was funking ugly so I donated it to some beggar in Windhelm. He wasn't interested though in a bunch of bones and skin, beggars these days man...