The item Odd Keystone is vital for catching a Spiritomb. There's one you get from a black belt on Route 209 (is it?) and one in Twinleaf Town. Just use Surf near the pond there, surf to the trees on the left, press A and you'll find an Odd Keystone there. If you want to find one the hard way, look for one in the Sinnoh Underground.

This cheat founded by Alex, posted by me!