The Elegant Suit can be bought from the tailor in Thieves' Landing for $70, but only after you've taken part in a poker game in any town. Once you've bought it from the tailor (whose shop is in the North part of town, on the East side of the circle - look for the scissors icon), go into your kit menu and use the elegant suit in order to make it accessible from your safe houses - the trophy popped just after I did this.