Is it possible to make a jam-setting gelling agent at home? Many jam and marmalade recipes call for a gelling agent in order to solidify the fruit-sugar mixture. While some fruits (like apples) may contain enough of a gelling agent (pectin) to solidify themselves while cooked for a long time I wonder what I can do with other fruits if I don't have a gelling agent at hand (they are quite hard to get a hold of where I live).

I read about adding orange, lemon or apple peel or cores as well as boiling down apples in order to extract the pectin but I'm curious if there are other, maybe more foolproof and accurate ways, to imitate a gelling agent (pectin or anything else which works for a jam).
The book Putting Food By (Hertzberg) has a recipe:10 lbs. apples yields 1 pint pectin. Seems jam &amp; jelly specific.