You can get them several places. CP Performance, Keith Eickert, Teague, Diamond.
http://www.keitheickert.com/c-766-reservoir-tanks.aspx

Since were on the subject...

Has any one seen or know where to get a larger reservoir for a barnett trim tab pump? The issue is mine (stock one) dosent hold enough oil for the 2 hydraulic cylinders that lift up my hatch.

Mine are made by get real and they are amazing. The quality is top notch. I just really wanted to clean up the engine bay and since I have a single engine boat the triple worked perfect.
I bet if you called get real they would be able to make you one with a larger tank.
There power steering and drive reservoirs are super nice too.

[IMG]http://i1069.photobucket.com/albums/...w/P1010207.jpg[/IMG]

http://getrealperformance.com/marine...eProducts5.php