The same mudflaps fit the 1995-2000 LS400. A Kansas City Lexus dealer parts man told me that mudflaps were dropped from the Lexus accessory catalog in the late 1990s.
I bought a set from Metro Lexus Toyota in Victoria BC for my 2000 LS400 in 2003. Metro has an export license and can ship to the US. Here is their website:
http://www.metrolexusvictoria.com/index.htm The person in the parts department who helped me back then was Paul.
If you can not get them from a Canadian dealer, you should know that mudflaps were standard on all years of the LS400 in the UK and Europe. Before mudflaps were available from Lexus in the US, I bought a set in 1990 for my first Lexus, a 1990 LS400, from Lexus Park Lane in London:
http://www.lexus.co.uk/dealers/5CE1DFA6-6D1F-11D4-B5ED-000629CECDBF/ Lexus Park Lane also ships to the US ... or at least they did in the early 1990s.
Lexus mudflaps can get pretty expensive if you order from a Canadian or UK dealer. In 2003, the set for my 2000 LS400 cost CN$457.96 I still have the invoice.