What size are you looking for? I'm planning on ordering some small black split rings from Cabelas in the next couple of days, and since the minimum quantity is 144, I'll certainly have extra. I'm getting the #8 size and I'm not sure of the exact measurements, but the next size down (#7) is about 5/16" ID, so I'd guess the #8 rings are about 3/8" ID. If you want some just PM me with your address and I'll drop 20-25 in an envelope to you when I get them.
I'm not aware of any colors available that would match natural HA-III finish. The colors I commonly see for small split rings is silver(both polished and matte), gold, copper, red, and black. Black would probably match the HA-III finish the best and that's what I'm getting.
-Keith