Which rear mech to replace sram X-9 10 speed 2x10
You need to purchase a SRAM 10 speed rear derailleur to ensure compatibility with your shifters. SRAM previously made a wide range of derailleurs for X5, X7, X9, X0 and XX but have limited their range to GX and X0 following the market shift to 11 speed drive trains. 

GX has replaced the old X7 and X9 price points.

Your options are limited as SRAM and Shimano rear derailleurs/shifters are not compatible.