Taking the list of YC-funded companies from wikipedia, excluding those marked as defunct or sold, resolving www.${domain} to an IP address, and then looking that up via whois, I see:
Amazon.com (presumably EC2?) -- 2
Layered Technologies, Inc. -- 2
NoZone, Inc. -- 2
Rackspace.com, Ltd. -- 2
Software Technologies Inc. -- 2
BitPusher, LLC -- 1
Carnegie Mellon University -- 1
Columbus Network Access Point, Inc. -- 1
Global Netoptex, Inc -- 1
ThePlanet.com Internet Services, Inc. -- 1
So it looks like YC-funded companies usually have dedicated servers, but there's no single place where they tend to congregate.