I once worked for a guy once who claimed he could sole-source a peanut if he had to. Many of them boiled down to colluding with the supplier to find a set of properties that were unique to their particular peanuts; things like soil pH, average temperature of the area they were grown in, not too much rainfall, not too little rainfall. Most purchasers were neither educated sufficiently to question these kinds of specifications, nor were they particularly interested; they also wanted to find the path of least resistance through the process. We probably could have bought most anything we wanted. Later we had a DOE grant get audited and there was some nervousness on our parts about some of these shenanigans that might be found. The guy just chuckled and said nearly everyone has to play this game to some extent. His major complaint about our books was that our overhead costs were above average, and he helped us make a case to the Uni for lowering it. Didn't see that one coming.
For an item like chalk though, my employer now has an arrangement with one of the big office-supply companies who specialize in tolerating (and charging extra for) gov't nonsense. Lately it's been CDW-G, I think. They provide a walled garden of approved products (which don't include the kind of good quality chalk in question) and a semi-streamlined purchasing process. It works great for the purchasing dep't, I'm sure, but it sucks for end-users because it introduces the same kind of uncertainty that a purchasing organization is supposed to fix. For example, the prices quoted on the website are not the actual prices of the products, and the products have variable discounts; this and the latency inherent in dealing with the purchasing process makes budgeting and price-shopping difficult to impossible. In my very small dep't we usually have budget projections rounded to the nearest hundred even for small items like chalk/dry erase markers because doing otherwise is a waste of time. We spent a lot of time a few years ago, working around the purchasing system just trying to get decent chalk before fatigue eventually set in and we just gave up and installed whiteboards.