I know it's a lot easier when a recipe calls for "stock" to chuck in a couple of bouillion cubes and some water. Or worse, to purchase overpriced, ready-made stock from the store.
At the risk of sounding like Slave Galley Kitchen rather than Casual Kitchen, I cannot allow you to take this particular shortcut anymore. It is borderline unethical. Your cuisine will suffer for it, and you'll be ingesting excess salt. And for what--a little expedience?
The only time I'll guiltily indulge in expedient behavior like this is if I'm really, severely pressed for time and I need to crank out a dinner pronto. And even then I can't bring myself to buy ready-made stock. Instead, what I'll do is use one cube of bouillion per two cups of water for a milder, less salty substitute.
But when you do have some time, try making your own stock. Enjoy how it makes your dishes richer and tastier and yet adds no sodium whatsoever. Moreover, it's so easy to make a huge batch and freeze it up in ready-made containers that you'll never want to go back to the salt-laden pre-made stuff.
Put into a large stock pot:
2-3 onions, quartered, leave the papers/peelings on (yes, you read that right--leave 'em on)
Several cloves garlic, smashed or cut in half (again, leave the papers/peelings on)
2-3 whole celery sticks
Any vegetable trimmings or peelings from whatever recipe you are serving
All of the shrimp shells from the recipe you're using, or use a few shrimp cut into pieces, shells and all (can also substitute pieces of fish or other seafood, or even oyster sauce if you're desperate)
Fill stock pot with cold water, add more than enough water to cover the all the stock ingredients already in the pot.
Bring water to a boil on high heat, then turn heat to medium low, set lid slightly askew, and simmer for a minimum of four hours, but as much as eight hours if you have time.
After the stock has simmered, strain it into measured amounts and freeze for future use.
Note that even though this is a stock recipe specifically for seafood, it can easily serve as a fundamental base for any kind of stock.
For vegetable stock, just leave out the seafood (duh). For beef or chicken stock, just toss in some tailings, skin, fat or bones from the meat you're using from your recipe. You can also brown any beef or chicken bones and meat in a non-stick skillet before adding it into the pot. Browning them this way will give the stock an even richer taste.
A few final observations:
1) First of all, I'll never forget the first time I tried to make stock this way. I was thinking, "seriously, you just hack the onions and garlic into a few pieces and toss 'em in the pot? You mean you just leave the peelings on?"
It seemed so.... so irreverent. But that's exactly the way to do it. Leaving the peelings and papers on actually adds to the flavor, and it saves you from having to do any prep work (who said there's no such thing as a free lunch?). Thus putting everything together literally takes two minutes.
2) Note that I don't add any spices or seasonings to the stock. No black pepper, no salt (obviously) and no cayenne pepper. The point here isn't to make something spicy or heavily flavorful. The recipe you'll use the stock in will contain its own spices. The stock is supposed to be a vehicle, nothing more.
3) This is one of the few times when you get to bend the rule to stay near the kitchen. Also, fortunately, this is a dish that scales well. Use your biggest pot and make as much stock as you feel you have room for in your freezer.
4) When I said using ready-made stock or bouillion was borderline unethical, I was just exaggerating for effect. Indulge me--I need to say things like this once in a while in order to stand out in blogland! So, if you've used ready-made stock or bouillion in the past, or if you choose to do it at some point in the future, you are not a bad person. Well, not that bad a person. :)