Hey Pie,
I found this thread while desperately searching for the Kong brand rings. I have the exact opposite problem to you!
We bought McKinley the yellow rings when he was a pup, but when they became hard to find, we tried the kong rings. He's absolutely obsessed with them as they are 'walk only' toys and he will chase them till he can barely walk anymore if we let him....but Kong don't seem to be making them anymore and ours are falling apart - they always get a weak point and snap somewhere along the ring. We've taken to taping our remaining two rings back together as it seems to be the only toy he'll chase.
I've still got an almost new yellow ring zinger and thrower. We tried to get him excited over that, but without luck. I've been looking everywhere for the blue kong rings, but no go. Does anywhere here know where they are still for sale?
I'd be willing to look at a postal swap of our yellow ring for your blue ones, if you're interested!