There are a few free sound libraries available for download from the following sources. You can use these to build racks for Synfire and/or for use in your DAW.
If you want to set up a global rack quickly, please read this article first.
Free NI Kontakt Player
Native Instruments* Kontakt Player and the associated Factory Selection sample library can also be used to set up a global rack for Synfire in a few minutes (the entire download is about 1.2 GB, so depending on your bandwidth, this may take longer). If you already own NI Kontakt, you should probably not install Kontakt Player, as this would only clutter your hard drive with duplicate sounds and plug-ins you don't need.
Download and install the free Kontakt Player 5 and also the Kontakt Factory Selection library here: https://www.native-instruments.com/de/products/komplete/samplers/kontakt-5-player/free-download/.
Free GM Instruments for Kontakt
You'll find more drum kits and even a complete GM set of instruments for Kontakt here: http://bigcatinstruments.blogspot.de/2014/08/gm-midi-instruments-for-kontakt.html. The license does not permit Cognitone to use these free samples for commercial purposes, so you will need to download and add them to your rack yourself.
Free GM Drum Kit
Unfortunately, Native Instruments uses a proprietary note mapping for their drum kits, not the widely used GM mapping. This makes them somewhat incompatible with the GM-based example patterns included with Synfire, as well as many libraries and construction kits available on the Internet.
There is a free drum kit available from Analogue Drums that loads into Kontakt Player. It's GM-based and sounds absolutely great, although it's a classic physical drum kit only (no claps, percussions, electronic snares, etc): http://www.analoguedrums.com/details-bm.php
Copy the downloaded files to where you store your other sample libraries and drag the downloaded NI Instrument file (.nki) on a new Kontakt Player instance in your rack. Create a device description from the rack module's menu and set the category of the sound on channel 1 to "Drum Kit".
*) Trademarks: UVI, UVI Workstation, Native Instruments, Kontakt, Kontakt Player and Factory Selection Library are trademarks of Native Instruments, Berlin and UVI, France respectively. Cognitone is not affiliated with or endorsed by Native Instruments or UVI in any way. The above mentioned resources are made available to the public by Native Instruments and UVI. We provide these tipps and links for your convenience only.