It vanished at some point in the past week or two. It had started to crack, and it probably just fell apart and fell through the engine down to the street. Gone!
So where do I get another one? Is it a “one size fits all” type thing that I can pick up from an auto parts store, or is this something that I have to special order from the manufacturer (pontiac) for like $50 (in which case, I’ll just cover the hole with duct tape or something)?
These are not universal. However, if it’s a common design, e.g. a GM type that was used on many models for many years, it might be available in the aftermarket. If the car’s not too old, it will be available from a dealer, though it’s possible it’s not serviced separately (i.e., only comes with the reservoir). It’s surely available at some salvage yards.
Your local Pep Boys or other aftermarket auto supply may carry the ‘Help’ line of parts made to OE specifications. Check the catalog for an exact replacement. Next bet if that doesn’t work out is a salvage yard.
You could try measuring the opening, and then go to the grocery store with a tape measure and find some container with the same size lid. There are a wide range of container sizes there, and almost all of them sell for less than $5 (including the food).