What other goods is the US Gov the sole buyer of, for which the sellers can't easily shift to other customers? Cop cars clearly doesn't apply, as carmakers have

What other goods is the US Gov the sole buyer of, for which the sellers can't easily shift to other customers? Cop cars clearly doesn't apply, as carmakers have other customers. Doctors, hospitals, and pharmaceuticals will face true monopsony, which means sell at gov rates or quit entirely. Some suppliers will quit entirely, but most won't.