If you're looking for the actual trim parts, they're very hard to come by. I've only ever seen one place selling a repro of it, and it was inordinately expensive - like, more expensive than a replacement seat. Can't for the life of me remember where I saw it, but it was the kind of place that caters to the type of crowd that want to do show quality rebuilds of sandcast CB750's.
There's a bunch of alternatives on eBay that all look pretty much the same; they're like the stuff at the bottom of this page - P&K British seat covers and trims. Nope, I have no idea why that's listed where it is, but TBH the website is a bit of a shambles. I've used their spike strips before and they're pretty decent for what they are. This is their video of how to fit it and make it look right.
Unless you're going for something show quality, that trim should be just fine.