Good luck - hopefully SBO has what you need. But if your H36 has the fill caps that are discontinued, you'll have to change the fill neck and rebed to your deck. Honestly, as much trouble as that is I often wondered if getting a local machine shop to fabricate a replacement cap might be less work - no idea of the expense though. If cheap enough I'd get a couple made. You could use one of your other still existing fill caps as a template for them, if they'd do it.
If you do end up replacing the neck and rebedding with something that has replacement fill caps available I'll offer some advice. The spring loaded pop-up caps below are prone to failure. On the MH-40 I bought last fall all 5 of the spring loaded fill caps on water, sewage and diesel had failed in either the up or down position. I wrote the manufacturer, Whitecap industries, and they wrote back that the caps do not come apart to allow you to replace the spring inside. That's a shame, as they're $46 each plus shipping just for the replacement cap. It seems likely that corrosion from salt water destroyed the spring that actuates the pop up plunger. They all failed within 8 seasons of being new. As you can see, failing in the down position makes it really difficult to open the cap, or tighten it securely. Failing in the up position makes it a tripping hazard. In the spirit of full disclosure the exchange I had with Whitecap was with the CEO, and he replaced all 5 caps for free, so I came away a happy customer. But YMMV, so think that through. At least the replacement fill caps are widely available, even from West Marine, though they come in two sizes, so be careful.