You must be thinking of the single core line that came on a flat orange-colour card? I've had a quick look and it does seem that it's difficult to get, and I've read a few reviews that the likes of Draper who used to make it, have now changed to braided line and the brickies are moaning!
I've always preferred the braided line, even the old white type, as I used to splice it when broken instead of tying a knot. My mate hates the braided line and says it's too thick. But I know what you mean about pulling it tight. The single core line is a lot better for that.
Anyway, I think this is the one you want - my local independent tool shop sells it, although it says braided its the thin one.
Braided line resists fraying, is stronger than other lines & has less stretch than other lines. Line is stored on a useful easy to use card system. Made in Britain.
www.toolmanyardley.co.uk