I've found "OK" oar locks on Amazon after busting an oar lock rowing my dinghy. They are not substantial.
I think the solution could be a slice of steel pipe welded to the head of a bolt with a pin or clip in the bottom. I would smooth the sharp edges with a file or die grinder.
Stainless steel would be nice. It will polish nicely and hold up to the elements.
K Surplus in National City is a good place to scrounge for materials.
http://www.ksurplus.com/
EDIT: I was at K Surplus about a week ago and noticed some large shackles that could be cut down to shape:
http://www.ksurplus.com/ties_cables_chains.html
The shackles I saw were "jumbo" size, meaning oar size ... maybe from the ship yards. Not what they have on their website list.
Maybe the shackle pins could be ground down flush to make the oar lock pin, already with a hole at the end for a retaining clip.