http://www.teamfortress.tv/36394/thalash-xhair
http://www.teamfortress.tv/42192/thalash-crosshair-w-1024-h-768
download links in those threads. Your options:
HUD crosshairs
If your custom HUD already has a seeker crosshair, enable it, then use crosshair5 in-game.
If not there's instructions and downloads in the threads for installing a custom font with the seeker crosshair in it
VTF crosshairs
very easy if you've worked with VTF crosshairs before. Download to VTFs also in those threads
If you're not familiar with these it can take a bit to figure out but I can recommend using something like Leth's crosshair pack as base or cfg.tf's crosshair pack maker. you just have to switch around the files' name so it uses Thalash' crosshair instead
edit: Archie's post below should work. Follow his instructions. It's the VTF crosshair method I wrote about. You then have to use the command cl_crosshair_file "" in console (or go to Options > Multiplayer > Crosshair None)