http://content.penturners.org/library/general_reference/taps_dies_kitless.pdf
Probably your best bet is 6mm. but it depends on the size of the pen you want to make.
Classicnib.com - Bock
Meisternib.com -Jowo
exoticblanks.com - Heritance
richardlgreenwald.com -Schmidt.
you will need a tap guide as well as a die holder,
Fowler Micro Tap Guide 52 490 100 0 | eBay