You can make one yourself for about $1.95.
I took a defunct 1 quart-sized garden sprayer (broken nozzle) and attached a bit of hose. Then attached a brass hose barb (the $1.95 part) to an old, plastic reservoir cap. The nice thing is that Rover never changed cap diameters. The same cap that fits the Series II "soup can" reservoir, also fits the Series III dual master reservoir AND the clutch master, as well as both the clutch and brake reservoirs on the '96 Disco.
That's multitasking....
Cheers