Here is a quick tip for finding WinPE drivers for Dell Servers. You also find an additional tip in the end of the post that can help you find drivers for Windows Server 2012 R2.
Update 2013-12-16: In addition to the steps last steps, and thanks Mirko Schnellbach (@MadMirko) for tweeting this, you can also find Broadcom drivers support for Windows Server 2012 R2 here: http://www.dell.com/support/drivers/us/en/04/DriverDetails/Product/poweredge-r810?driverId=7TH1H&osCode=MWS80&fileId=3325762022&languageCode=EN&categoryId=NI
WinPE Drivers for Dell PowerEdge R820
Here is an example with the Dell PowerEdge R820 server, but no worries, the download contains drivers for most Dell servers.
Navigate to http://www.dell.com/support/drivers/us/en/19/Product/poweredge-r820, expand the Systems Management section, and download the Dell OpenManage Deployment Toolkit (Windows) v4.3 package.
After downloading, simply extract the package, and you find the WinPE drivers in the Dell\x64\Drivers folder.
If you can't find the driver you need in the Dell OpenManage Deployment Toolkit (Windows) v4.3 package, you can check if it's available in the Dell OS Driver Pack, available in the Drivers for OS Deployment section.
After downloading the package, you need to extract it twice, first you extract the main package, and then you extract the Dell\payload\mas021.zip file that has the drivers.
After extracting the Dell\payload\mas021.zip file, you can open the drivers.xml to find the right driver. Here is an example with a Intel Nic driver for Windows Server 2012 R2.