They are called aeroports. Lightning audio(car audio manufacturer) carries them. Try local audio shops first, if you have no luck there, ThomasW found a online vendor that sells them. Sorry I don't have the website address
ThomasW will know exactly, as it's tough for me to measure, but I think it's right around 9", and there's a 1/4" flange, so the max flare diameter is 9.5".
Here's where I got mine: Link Removed
That is the through-hole diameter. These things sit kinda weird, there are protruding pieces around the underside that hold it in place, and they need about 9" to fit, which doesn't leave much of a flange for glue or other mounting.