I wish I could be more specific, but I want to think because of the "new interest in rimfires" supposedly brought about by the HMR, that someone was being courted to start producing 5mm rimfire ammo again. I know this doesn't help any, but maybe give you some encouragement to keep looking around. Maybe it's out there by now. Otherwise, you'll probably have to keep an eye on gunbroker. If you have any local gun shops, ask if they know of any sources.
It's in the old-school red and white box NOT the current green and yellow box. There are 50 rounds per box. It belongs to my father-in-law but he wants me to sell it for him. He's not internet saavy. Email me directly @ [email protected] and I will send you pictures of it for you to see. We can work out something price-wise if you like what you see.