as the topic says. is it possible to craft the metal tubes? Because trader is deactivated on my server.. unfortunately.

We have found metal tubes in Grushnoye in the red metal buildings.

Good to know want to set up some Watchtowers, but didnt found them so far.
are there on the top of the cat walks in those metal buildings?
Or any other known locations for those metal tubes?

I have found them on top of the oil storage silos

It seems as though they are in the same places you might find hydraulic pipes.
Cat walks etc.

Don't know about catwalks, but the best place to farm those tubes is Tarakanovsk Oil Storage to the north of Apshironsk. There are 15 oil tanks.

Using silenced guns to get rid of zombies saves a lot of time there. Before going up those tank stairs you can go on top of the near construction site and easily look over those tank roofs for any valuable stuff. And it's also better to have a small helicopter so when all the roofs are kinda full of waste loot (two spots per tank) you can hop in the chopper and quickly fly over the cisterns to refresh the whole place and – what's the heli for – see if the place is actually refreshed, because sometimes it takes more then one overfly to do the thing.

So, let me tell you, that's the best way to loot the tubes, if you're of course willing to spent some time on farming them. As far as I know, different servers have different settings as for the loot spawn, but it works for me so I find one (less two) pile of tubes per hour.