Mine wasn't leaking where you say yours was, but you may be interested to know that CARQUEST carries a huge kit FULL of all sizes of orings that they will individually sell you (= cheap). There are 4 orings to the item you want to replace and I recently dug to find out if anyone carries them and the search resulted in no finds. So, either you would have to go to a junkyard or take the valve completely apart and replace the internal oring that I assume is leaking. And I say 4 because I think I remember during my search someone on here said there were. My Helms manual didn't have an exploded view and mine was leaking from an external oring, so my disassembly stopped there. Hopefully it's not too hard to get to the internal ones. Hopefully just a snap ring? I'm sure someone else will add or you can search on how to disassemble the valve to get to the bad orings. Then take bad orings to carquest and match them up. FYI, my CARQUEST didn't have a price listed so the guy gave me two orings free.
Just thought of this too....you SURE it's leaking between what you said? Because it may just look that way and if it's leaking further toward the pump (between the power steering reservoir and the valve) and gravity is letting the leak travel underneath the larger outside diameter, you would have a much easier fix!
Good luck!