They come as a package, I’ve never seen the trousers on sale on their own o.O
Any clothing shop will have them, Debenhams, Next, Matalan is a good shout actually try there, you’ll even find them places like George. Anywhere that sells night clothes really.
Just pick cotton rather than polyester for your pajamas, much more comfy :-)
I wish there were another answer, I’ve wondered similar things myself for many years and therapy is always the solution everyone suggests.
The problem is, where I live therapy is only available to wealthy people. It’s not something I’ll ever have access to :-(