There are several options.
Start by researching on PWC Moneytree and CrunchBase about investors who have funded marketplaces (e.g. Benchmark has invested in several over the years.)
BTW, Greylock just announced (end-Aug 2013) $100M marketplaces fund.
From my conversations, several investors seem to be looking at the marketplaces theme these days.
Happy to talk more, if you'd like to.