Almost any starch should sub for the yam starch. Kudzu, arrow root, tapioca, even flour in a pinch. After all its basically just an omelet or eggy pancake. As for the bonito flake (and it is bonito) can probably be subbed for any dried fish product. Though the texture will be quite different. Or it can easily be found online. Fish sauce doesn’t tastes nearly the same, and it will soak into the pancake changing the flavor quite a bit. After that most of the exotic stuff can be found online. Okonomoyaki sauce is the exact same (or nearly) as Tonkatsu sauce, and you can make a suitable version at home:
And Japanese style mayonaise is incredibly easy to make. Just make some fresh mayonnaise using just egg yolks no whites, cider vinegar for the acid, and sprinkle in a bit of MSG (this is critical, its the major reason Japanese mayo tastes so different).
Most supermarkets or specialty markets should have sheets of nori for sushi you can toast and chop up.
After that everything is just normal stuff you can find anywhere. Its basically those toppings that are harder to find, and I’ve seen okonomiyaki done without either/both bonito and seaweed.
I’d go easy on the bonito anyway, its pretty strongly flavored so it can overwhelm everything else. Its also pretty dry and hard to chew so too much is a textural problem. The same batter can be used for takoyaki if its heavy enough on starch. Just use an aebleskiver or similar pan.
Not all of us live in places that are anywhere an Asian grocery. I’m in the NYC metro area and I still need to drive an hour plus to get to an Asian market. And that market will be pretty rigidly Chinese. They carry a lot of dried Ramen, Japanese Condiments, and prepackaged curry mix. But that’s it on Japanese ingredients. No bonito, no Japanese seaweed products. I tend to wait till I’m in Queens visiting friends to load up on Asian and Irish products. I’ve got family in the rural south, and central Maine. The one time we checked down south the nearest Asian market was a few hundred miles away. In Maine I suspect it would be even further. I’ve never even seen a white person with a tan in my mother’s home town. My great Aunts and Uncles still look at Italians as suspicious non-white intruders, and they don’t even have Chinese take out places. Or vegetables beyond iceberg lettuce apparently.
It was initially described to me as a omelet, but its certainly more pancake like in a lot of ways.