Here's a link to the EagleCAD files for a QFN48 socket to J-Link/JTAG 20-pin connector to program bootloaders, etc for Nordic nRF51:
I included a USB connector as an alternative to using the 5V power from the J-Link, but haven't had to use that yet; the power from the J-Link device works great!
I also included a 16MHz crystal in the design so that I could boot up the nRF51 and do some basic validation. Probably could get away without populating those or related components.
The socket is available on Amazon.com and Ebay.com for about $100 it's a QFN-48(52)BT-0.4-01 manufactured by Enplas.