Every chef worth their salt makes their own.
Sometimes though, that friendly little tetra-carton means one less thing to wash and far more space in your freezer.
I do both, but have to say when it comes to speediness, that little liquid-filled box is my go-to man. However, I do have a little bag in the freezer in which I collect onion odds and ends, carrot tips, mushroom stalks, whatever's going.
My usual list is onion, celery (perfect for the leaves at the top), carrot, mushroom, bay leaf, salt, pepper, garlic cloves, herbs and old lettuce. Don't have anything too starchy (potatoes, etc) or anything too cruciferous (broccoli, cauliflower).
Just chop them roughly, quarter the onions and carrots, and throw them in a big stock pot.
Pour over some water almost to the top, add sea salt and cracked black pepper, and simmer for as long as you want. The longer the better!
Cool for a bit and then either use or freeze for another day.
Don't forget to keep your bags in the freezer ready for your scraps, and you can just throw them into the pot frozen as-is. Easy!