If you can't find any long enough you can put 2 together. Not the ideal I know but it works.
Ahhhh, just checked my favorite on line supplier- McMaster-Carr .... 316 stainless in up to 12 inches
McMaster-Carr
I just put 2 together as well. Works just fine. The inner tube covers the second screw mechanism anyway so nobody will know but you.
Jim