Wardrobe and description from ScreenUsed.com:
"This is the suit worn by Will Ferrell as Frank on screen during Blue's Funeral where he sings 'Dust in the Wind'. The costume consists of pants, shirt, and jacket and each piece has 'Frank' written on the inside in ink."
Following his rendition of Kansas' "Dust in the Wind," Frank iconically shouts, "You're my boy, Blue!"