get a decent pair of cycling bibs w/chamois (trust me on the bibs instead of shorts). put them on first (no underwear) then any pair of shorts over top.
or there are shorts like this that have a liner. I have never used these but they do exist and may work well for you.
Nashbar Liquicell Sport Baggy Shorts - Shorts
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2010 Kestrel RT900SL, 800k carbon, chorus/record, speedplay, zonda
2000 litespeed Unicoi Ti, XTR,XT, Campy crank, time atac, carbon forks