Putty Tape

I am a little confused on this. What specific brand do you use and where can I find it? I have looked at the big box stores and the stuff they have looks like it has a finished side for around tubs and sinks. It also only has about 10ft in a roll. I am guessing this is not the stuff I want. I have also seen it referred to as Butyl tape, caulk tape and putty tape. I saw a thread a few days ago stating that Putty tape is the stuff to use because it maintains the same thickness off the roll rather than getting thin spots. Thank you for any clarification.
I am getting excited - I am meeting with the welder to start the trailer build this week and the aluminum comes in on the 20th! WooHoo! I have attached a few pics of the progress so far. Thanks again to everyone.

I am getting excited - I am meeting with the welder to start the trailer build this week and the aluminum comes in on the 20th! WooHoo! I have attached a few pics of the progress so far. Thanks again to everyone.