Where to find the source file of the header netdevice.h

Where to find the source file of the header netdevice.h

"cscope" is a very simple to use command line tool. you will have to install it and then use it on the kernel source tree locally. (it will index) However, if you are comfortable with the web version, that's also fine. I find cscope a faster way, so suggested that to you. :) the mechanism hardly matters.
Just as an e.g. (I used cscope)
Look at the file: drivers/net/82596.c
 line: 1125  
 static const struct net_device_ops i596_netdev_ops = {  
 .ndo_open = i596_open,  
 .ndo_stop = i596_close,  
 .ndo_start_xmit = i596_start_xmit,  
 .ndo_set_multicast_list = set_multicast_list,  
 .ndo_tx_timeout = i596_tx_timeout,  
 .ndo_change_mtu = eth_change_mtu,  
 .ndo_set_mac_address = eth_mac_addr,  
 .ndo_validate_addr = eth_validate_addr,  
 };  

These are the driver routines for the particular device. Now search for these function definitions in the kernel source tree