I know what enchant (Pat) is talking about. I cannot find instant dry yeast sold by that designation, or simply as "IDY", in the supermarkets near me, and I look at the yeast sections all the time. It is typically sold as bread machine yeast, which is usually supplemented with ascorbic acid (Vitamin C). There is also "RapidRise" ("instant active dry yeast") or "Quik-Rise" yeast ("fast acting yeast"), which arguably is a species of IDY but not sold as such. I once tried to get Fleischmann's to tell me whether the RapidRise yeast was an instant dry yeast and got only ambiguous answers and links to recipes. But after much prodding, I was told that it was not identical to instant dry yeast.
The yeast producers who sell IDY reserve that designation for professionals. The websites of the major yeast producers like SAF, Fleischmann's and Red Star tend to be consumer oriented and, from my research in the past, don't use the exact expression "instant dry yeast" or "IDY". You will find those designations on the products sold to professionals. Those are the products that are sold by the big box stores like Sam's, Costco's, etc.
If enchant stored his IDY in the refrigerator and is looking for a replacement, I would either buy a new bag or try one of the bread machine yeasts.