Can you not get it through a Grady dealer? If it's something that Grady made themselves (or had made special for them), that's likely your only "quick" source. Like most boat manufacturer's, part numbers are internal. It's not like car parts where there are external databases and retailers for this. Other than that, probably just keep searching eBay.
I do seem to remember another post about this question from maybe a few weeks ago? Search for that and see what the author of that post found.