From the US? A huge choice of costumes is available at Amazon.com
Despite being a lot cheaper to buy, this Super Workman costume featured above (by Henbrandt) has been receiving pretty positive reviews and is also available in green which is ideal for dressing-up as Mario's brother, Luigi. The outfit fits up to 46" chest and 36" waist, and includes the dungarees, top and hat, although the white gloves and moustache are not included so you'll need to buy them separately. Order before 12pm and the costume will be despatched the same day.
This Deluxe Official Mario Costume is available direct from Nintendo's store on Amazon and actually is actually great value compared to other offerings. It even includes an inflatable tummy!
This moustache is the best-rated on Amazon and is perfect for the Mario look.