How much do you want? I would not be convinced that this length would be held by a stockist, but a mill would probably make several kilometres for you.
It would also help if you gave us an idea where in the world you are, and what type of CS pipe you are looking for API 5L, A53 or just spiral welded.
We commonly buy steel pipe (ASTM A53 Grade B) in 20' ("random") or 40' ("double random") lengths. The "random" means there is some permissible variation in those lengths. I've not seen pipe sold in 60' lengths, but can't say it's not out there. Structural steel is generally available in 60' lengths.
The resom most pipes come in 20 and 40 foot lengths is it fits on a standard trailer. 60 footers is a oversize load. If the cost of the oversize transportation is less than the cost of the two field welds, I'd go for it.
If you are real close to a railyard, it may be even cheaper to get the longest pipe that can be put on railcars. Then local trucking to job site. I think you can get 80 footers on rail.
If you're buying enough line pipe (unfortnately, 3.5km isn't enough) the steel mill will give you whatever lengths you want- I bought linepipe in 15m (50ft) lengths from Mannesmann once, as that was the maximum length we could get on the local roads.