If you dont want to make it from scratch marigold swiss vegetable bouillon is very good.
I use, I stick of celery, 2 carrots, 2 onions, a bayleaf, pepper and salt.3/4 litre of water. I chop the ingredients roughly, chuck them all in the pan and boily with the lid on for 30 - 40 minutes. Then strain it through a sieve, I push some of the veg through the seive as well.
reduce it til you have a very dark and small amount of liqour - allow to cool, pour into an ice cube tray and freeze - makes your own stock cubes for the future - use up within a couple of months...