If the texture you want to tinker with is not in-game, try
here.
If it is, go to tf/tf2_textures_dir.vpk for the textures and tf2_misc_dir.vpk for the accompanying .vmt files.
I highly recommend you have GCFScape for opening .vpks and VTFEdit for editing and making .vtf files and .vmt files. Don't run Hammer in the background while you try to open a .vpk file or GCFScape will give errors.