I would like to install an auxiliary gas tank under the truck bed of my 2006 Tacoma. Ideally it would be integrated with the stock tank. I'm looking for a fabricator in the Southern California area.

 

it's not southern california, but if west coast oregon fits the bill there is a great aux tank fabricator called Northwest Metal Products. I have their 11.5gal auxiliary tank in my 2nd gen and love it!
it is a gravity feed into the main tank. needle doesn't move away from full for a looong time.
here is a link to search for your application

http://www.nwmp.com/search.asp

 

my mistake, I just read the fine print and saw that gravity feed tanks are illegal in california.
sorry about that.

Gravity Feed-Any of our fuel tanks which are referred to as "gravity feed" in simplified terms means that the auxiliary (or additional) tank is mounted in a higher position than the stock tank which allows the fuel to be gravity fed or flowed into the stock tank. The stock fuel guage in the vehicle will remain constantly full until the auxiliary tank has completely emptied at which time the stock tank will start to register as usual. Gravity Feed Tanks Not Legal For Sale Or Use In California On Any Pollution Controlled Motor Vehicle.