Might not be as cheap as I said, I don't actually remember. Not expensive though anyway.
I found some, but I was stupid and only bought one. And I was even stupider and dropped the hydrometer I bought so it broke.
I also bought bungs that fit water bottles. But I'm currently using a 5,1 liter water container with a bigger opening, and they didn't sell any fitting bungs for that one so I made a hole in it's original cap and fitted a smaller bung in it. Works perfectly.