Hey that's my picture!
I have no idea where you might find one other than asking Seek or someone who removed theirs & doesn't want it anymore. BUT consider this: If you lost the pin you have also likely lost the reference point of where the lens was able to focus while the pin was installed & you might not be able to get it back in the right place. The pin restricted the focus adjustment to one turn. But how far out was the lens when you removed the pin?
My experience is that removing the pin allowed the lens to move out one more turn. So if you are having trouble focusing without that pin in there, try this:
Turn the lens shroud fully counter-clockwise (not forcing it), then you will have it on very close focus. And about 2 turns clockwise from there should get you focused on distant objects. It has 2 or 3 more turns to go at that point before it hits bottom and if you go into that range you will just be more out of focus.
Another thing, is the pin actually lost or is it just pushed up inside the shroud?