where can i buy a 12" heavy duty adjustable hitch drop.

"Heavy Duty" and "adjustable" are mutually exclusive. I wouldn't tow with an adjustable for any reason but if I were going to, I wouldn't go over 2K/lb with it.

12" drop is ALOT. That adds a huge amount of leverage. I have one for my Excursion's WD hitch and it's only 9". It's a solid bar with a big gusset on the back.

my boat weighs 7500 lbs and it is on a alumunim trailer not sure what it weighs. to keep the trailer where it is level and will tow good i need about 12" drop. here is the truck.

Make sure its beefy.

I got mine from northshore trailers 1 800 272 0283 it's made of billet aluminum it's not cheap but they are good for 15000
i will try and upload a picture

check these out

http://www.jcwhitney.com/Ball-Mounts...2319_10101.jcw

The black one would be at it's maximum right out of the gate.
The show chrome isn't even a contender.

I'd find something non-adjustable that is rated for more than what you need - like the 10,000-12000lb unit someone mentioned.

I saw a billet aluminum hitch at Simpson Tool Box in Phoenix, AZ rated for 10 or 12K. I think they wanted about $170 you could call them. I also saw the same hitch at some other local trailer parts places.