Because I am into spending less and wasting little, I make my own stocks. During the warm months, I compost all my peelings and normal veggie parts one would toss into the trash. However, in the cold months, when I can not compost, I use those peelings to make my veggie broth.
Every time you peel, cut, etc, save those trimmings into a gallon sized bag. Make sure the veggies are cleaned and the trimmings are relatively dry. I pat them down with paper towels. You can also use a salad spinner if you have one. Store the trimmings in the fridge.
Some good trimmings to use:
Stems from herbs
Leafy celery tops
Potato peelings
Carrot peelings
Bell Pepper scraps (no seeds)
Any parts of an Onion
Tomato cores
Some bad trimmings to not use:
Broccoli
Cabbage
Cauliflower
Brussel Sprouts
Basically anything that is bitter, do not use.
After about 4 days, it is time to make your broth. Try to have about 3/4 of the bag full of trimmings. If not quite there yet, just add some limp celery or other veggies you have on hand that have seen better days.
I make mine in my crock pot and have it cooking on low over night for about 8-12 hours. Make sure to cover the veggies in water. In this batch I happen to have a bunch of potato peels and my gallon bag was full. I had to keep pushing my veggies down (they tend to float) to make sure I had plenty of water in the crock pot.
After the stock has cooked, strain & toss out the veggies. I strain the broth again through some cheese cloth to catch anything small that might have snuck through.
From here, I can the broth for a later use. Process pints for 30 minutes and quarts for 35 minutes at 10 pounds of pressure. Do not use the boiling water bath method, only the pressure canner is safe to can broth of any kind.