SeaLand has 'em...
They're under $10. But first you need to know whether your deck fitting is 1.5" ID (inner diameter) threaded, or 1.25"...then you need to count the threads per inch--that will be either 11 or 16 (11 is most common on newer boats, 16 on older boats--and btw, if yours happens to be 16, hang onto the cap! It's impossible to find a replacement for it any more). Then call SeaLand at 800-321-9886 and tell 'em what you have. In the unlikely event your deck fitting doesn't match ANY of the above, they do offer a neoprene "universal" nozzle...it doesn't thread in...you have to jam it in tight and then stand there and hold it while you pumpout.