Although I've never tried BTB, I've never had a pre-made/store-bought stock that I liked. So I try to keep frozen stock around.
When I don't have any stock, I generally use one of the following:
1. Plain water (it's better than bad stock)
2. Make a quick vegetable stock by simmering any vegetables (esp carrots, onion, and celery) in water for about 30 minutes.
3. Use wine, beer, vermouth, apple cider, etc., perhaps in combination with water.
Naturally, the best substitute depends on your application.