EpisparhNuma
That is more an EDDN glitch though. I've actually found at least the 8C and 8D in game, and their price was what you'd expect it to be, not 0. I'm looking for the 8A, which I actually want to buy. Seems to only appear in military ground settlements and on outposts, I had no luck so far though, they're ultra rare.
You can buy FSD as large as your ship can fit. Even if you find size 8 you cannot buy it.
In EDO you can "buy and store" any module, even if it doesn't match your current ship.
But since those class 8 FSDs were last seen by the EDDN more than a year ago, I doubt that they're still on sale. If anyone finds them, please post a screenshot.