This applies to vegetable stock, or any meat stock:
Save the scraps of whatever kind of stock you are making.
If you are making vegetable stock, save all of those scraps from vegetables that you won't eat such as onion and garlic peels, the leaves of celery if you don't eat them, literally any part of the vegetable that you will not eat.
If you are making chicken stock, save all of those bones and fat and any part of the chicken you will not eat. (Don't forget if you go out for fried chicken or chicken wings, don't throw that stuff away, RECYCLE)
You will want to keep these separate. Have a vegetable stock zip lock bag in the freezer, have another bag for chicken, another for beef, etc. Freeze this stuff until you are ready to make your stock.
When you have enough to make the stock, put the scraps in some water and add salt and any other spices you like.
If you have a crock pot, leave it overnight in the crock pot on the high setting.
If you do not have a crock pot, put it in a pot on the stove and bring the water to a boil. Reduce heat and simmer for about 1 hour.
Strain out the scraps and store the liquid in the refrigerator.
There you go, now not only do you not have to buy the stuff, but you will cut down on your food waste.
Also, if you have a garden, you can put the vegetables that you just made into a compost pile along with egg shells and expired canned good for double recycling. Do not, however, put the meat scraps and bones in your compost.