Are you looking to BUY or MAKE?
If you want to sew your own, try Truly Victorian Patterns. The owner, Heather, makes the easiest Victorian patterns available...she also has a forum with TONS of info. There might even be some Mrs. Lovett threads over there, too. If I'm not mistaken, they (the movie's costume designer) used one of Heather's patterns for one of Mrs Lovett's skirt.
dani