The map sources can be found in the data*.pk3 file. Take a look at the maps/nex*.map files. The weapons however can be changed via the maps/nex*.ent files too. If those files are present they 'overwrite' the information inside the compiled maps (maps/nex*.bsp files)
Its easier that way to change items but as radiant does not understand .ent files you can endup with diffent items in radiant and ingame. And thats bad.
You can change the start weapons via the g_start_weapon_* variables. Use tab completion or take a look at the wiki for the other names. Too lazy to copy them all

. You can use the .mapcfg files to save the current values and then use the exit config file to reset them to the old values. See maps/ctf_nextctf01.mapcfg and maps/ctf_nexctf01_exit.cfg for examples on how to do such a thing.