For Eric to get the T-Rex pee, I can only think of two things.

1. He saw the T-Rex while it was urinating and after it left he approached the puddle which was substantial enough to fill the flask.
2. He was out in the forest and could hear it approaching, hid on the ground and the T-Rex just happened to stand over him and Urinate all over him.

A big, yellow puddle.

I have a humorous thought on this. He came across a T-Rex and didn't move so it wouldn't see him, and it ended up pissing on him. On his way back to his "home", he notice that the smaller predators (like the compys) were avoiding him because of the smell, so he rung his clothes out and kept some for protection ("It keeps away most of the little ones, but it attracts one real big one with a fin").
Btw, scientists have discredited the If You Don't Move It Can't See You theory. Although this is true for several amphibians, of which the dinosaurs were spliced with, so whatever.

He said "you don't want to know" only because the screenwriters didn't know.

Consider the daffodil. And while you're doing that I'll be over here looking through your stuff.

He sneaked into a T rex nest with a glass of warm water LOL

He scooped it out of a plot hole.

I got the impression that a T-Rex showed up right above where Eric was hiding, then Eric swallowed the stuff, raced back to the truck and regurgitated it into the beaker.

One of the most important questions ever in Jurassic Park history! So after rewatching Jurassic park 3 VERY recently I came to the conclusion that Eric probably scooped up the pee with a bottle after the T rex left.
He only says "You don't wanna know" only because it is pretty disgusting to scoop up Pee. I kinda wish Jurassic World have small talks like these

Ha champagne showers

golden shower

It was seperate cus they have found t rex poo petrified with bones in it

This message has been deleted.

So he sucked off a t rex lol

Same way you put gasoline from one cars tank into another, heres what you need
car with full tank
a container
a rubber hose
suction
Put one end of the hose in the full tank, put the other end in your mouth and suck on it until the gasoline fills your mouth and quickly put the end in the empty container.

Collection
http://www.imdb.com/list/4zXrE3AAzT4/

He bought it at the Lazy Writer Store.

He gave it a plastic cup and told it to go use the Ladies' Room.

A straw

but this is exactly what happened?
the t - rex pee IS how he survived he just didnt get it by beeing ped on

If the T-Rex pissed on him that would help explain how he survived, other animals would avoid anything smelling like T-Rex tee tee.

or better yet the trex pissed on him while he's sleeping/hiding under tree groves

"you don't wanna know"
I suspect he found the t-rex having a jimmy, which, as said, would probably produce a fairly sizable puddle, soaked his t shirt in the pee, drained it into a flask back in his truckhouse.

Thoughts?
IIRC Erik told Grant when he inquired, "
You don't wanna know
" as to how he got it. Now of course, and without getting eaten, he surely did not stand underneath to collect, however I presume while wandering the island he came across a Rex - In not raising its attention Erik presumably froze/hid until the lizard was finished with its business. Given the size and girth of the animal (incl. its bladder) the result may have been an extremely large puddle, or mini-pond. When the Rex left and the coast clear, and being such a
brilliant
kid, he thought to fill a jar (which he happened to have with him at the time?) with the urine as a repellant, and or for use against other dinosaurs?
I dunno - I found the disc as it is well-worth watching again.

• DominicD
  "Always make the audience suffer as much as possible."
  Alfred Hitchcock