Hey all, I have various injuries (etc.) and one of the most helpful things to do is apply cold. So I've tested a lot of stuff.
This is what I've been using for the past five years. It's not quite perfect as it leaks slightly at the neck, but it is much, much more durable than other ice bags. I've only managed to break one of them and that was by freezing it nearly solid and then slamming it against a metal sink HARD. This made sharp fragments sturdy enough to get through the rubber. (The rubber should be thicker IMO to be truly BIFL.) But I'd already done the same thing many times without issue, so that's pretty incredible. As an added bonus, it can also be used for hot water. And it's cheap ($10).
It is incredibly useful to be able to pop a warm bag back in the freezer to refreeze, as anyone who needs this much icing down will understand. Ditto having a single, compact item that lasts for years and does both hot and cold. Most ice bags out there are cloth glazed with plastic, and when you freeze them the ice crystals rip holes in the plastic and then they're garbage. Those cannot be refrozen, and forget about using them for heat.
For a truly BIFL cold/heat therapy, get something like this. (Technically it's an enema bag, but in my house growing up, it was a "hot water bottle".) I haven't bothered trying it for ice, because I ice knees/shoulders and it's too flat to be effective (I use a combo of ice and water in my bags, allowing me to nearly encapsulate my aching joints).
They do still make new ones. My parents' one is 50 years old and still in use.