After it was suggested here that Postum is one of many "available" substitutes for high-priced coffee, readers began asking, "Available where?"

As one woman put it, "Postum has just about disappeared from the shelves of local supermarkets. Why? When will it be available?"

In an attempt to find out, I accompanied my wife on a marketing expedition that took us to three stores. I found in Postum at any of the three. The managers and assistant managers to whom I talked could offer no information as to why the product was not available or when it would be.

The next day, I called the corporate offices of several local chain stores. The basic response from all was the same: "General Foods has put us on allocation and we never know when we'll get our next shipment. When we get the stuff in, it disappears as fast as we put it on the sheleves."

Safeway's spokesman said it had been quite a while since that chain had recieved any Postum, and he was hoping a new shipment would arrive "any day now."

Joseph B. Danzansky, president of Giant Food, added some background. He said, "Even though we're on allocation now, we're selling four times as much Postum as we were a year ago. When a shipment comes in, we apportion it on an equal basis among all our stores and put it on the shelves. I think you'll find that all our competitors are doing the same thing. It's the only fair way to handle a situation of this kind."

Danzansky added that his buyers had been told by General Foods that steps were being taken to increase production, but there was no word on when the situation would right itself.

The next step, obviously, was to call General Foods in Newark. A woman there told me that my questions could be answered by a woman in the General Foods office in Kensington. The woman in Kensington asked if it would be all right if a public relations person in White Plains, N.Y., would call me. In due course, the White Plains spokeswoman called and gave me the information I needed.

She said that Postum sales began to spurt about two years ago when the product developed a strong following among people interested in health foods. A "dramatic" increase in sales followed when coffee prices began to take off into the stratosphere.

Capacity production at the factory just barely kept pace with incoming orders until around the first of this year, when demand completely outstripped supply and General Foods was forced to put its Postum customers on allocations based on their previous purchases.

How long will the shortage last? The spokeswoman said the factory is now making some production improvements that it hopes will permit it to end allocations within six months. No vast expansion is being planned, and nobody at General Foods is prepared to guarantee that supply will catch up with demand in those six months.

One doesn't have to be a business genius to understand why no vast new Postum factories are being built at the moment. When coffee prices begin to drop, as they will, some percentage of today's Postum drinkers will no doubt decide that they like the cereal drink well enough to stay with it. But eh remainder will switch back to coffee - and until General Foods find out what that percentage is, and where demand will level off, the company will be understandably cautious about building new factories.

But as Danzansky commented, "We have to give them credit for keeping the price down at a time when they might have jacked it up. Postum costs about a third as much as instant coffee, and that's going to make it attractive to some people, quite apart from considerations of taste, content, or health."

If the great Postum shortage eases, I'll try to keep you posted.