Legionnaire Greatsword
If I’m not mistaken, that’s actually available on the Trading Post under rare or exotic items. I’ve seen them from (I believe) level 50 through 80.
Bring your Trading Post menu up and look for the keyword “Legionnaire” under the greatsword filter and you should be able to find one for sale. I’ve seen Legionnaire’s Mauls up there, so good luck!
I think he means other than the trading post. Or at least I would like to know where they come from. Random drop? Crafted?