There’s some here: http://cpc.farnell.com/particle/jshield/spark-core-jtag-shield/dp/SC13388
And @Elco made one that’s a bit different if you can ask him to post the files, you can order those at OSHPark.
Hey look someone ordered and shared the original JTAG board on OSHPark: https://oshpark.com/shared_projects/60Ez1xCD
Edit: I just made a small JTAG Shield and ordered on OSHPark: https://oshpark.com/shared_projects/rMrGEUy3 (technically you don’t even need to stuff the 10k resistors) … this one uses a right angle 20-pin header:
http://www.digikey.com/product-detail/en/302-R201/ED10536-ND/2794247