I expect they get it out of the same national distribution pipelines as
everybody else does. Basically *all* brands of gasoline are exactly the
same base gas from the pipeline, the only difference is the additive
packages that the different brands add to that base gasoline when they
fill their delivery trucks.

Unless you are driving something with a high performance / high
compression engine I expect it makes little difference what brand you
use. I know my truck with the big block doesn't give a damn what crap I
put in it and hasn't for the last 163,000+ miles.

Pete C.

The same place everyone else in the area gets it from.

Gasoline is gasoline. The only difference is in the special additives.
All gasoline is mixed together in the shared pipelines. If Mobil puts
100,000 barrels in, they take a 100,000 barrels out from the other end.
Odds are it won't be the same gasoline they put in. Same with everyone
else. Cosco probably buys gasoline from someone, taking the amount
purchased from the end of the pipline. As it goes into the tanker truck,
the additives unique to the brand are added.

Gasoline is gasoline. Different beyond-the-minimum additive package at the
most.

Costco probably buys it from whoever gives them the best deal, like
Sam's Club/Walmart and other independents.

As for quality, there was a piece on Motorweek last week about 'top
tier' gas. They had a woman from Shell talking about how you should only
use 'top tier' gas in your car and showed a set of valves from a test
engine they ran in their lab. According to her, it was a V6 that they
had set up to run a 'top tier' gas in one cylinder bank and a 'lower
tier' gas in the other. After the equivalent of 5000 (or was it 3000?)
miles they tore the engine down and looked at the valves for deposits.
The three that came from the 'top tier' bank were clean as a whistle and
the three from the 'lower tier' bank were really grungy looking...huge
carbon deposits. So, as good old Pat Goss said...you certainly wouldn't
want to run your car on anything but 'top tier' gas, now, would you?
Kind of a thinly disguised commercial for Shell altho they never said
whether Shell was 'top tier' or 'lower tier'. :-) I run my cars on Sam's
Club gas...wonder which 'tier' it is???...

(They also slobbered all over themselves about the new Chevy
Silvarado...sounded like Shell/GM-week instead of Motorweek. Kind of
soured my opinion of Motorweek...)

They are supplied by an independent company. Could be just about anyones
base stock.

http://www.faqs.org/faqs/autos/gasoline-faq/part1/

I filled up at Costco in Georgia yesterday as a BP tanker truck was
unloading.

From my day job I know the distribution chain for this area and it all comes
from the Doraville, GA terminal served by Kinder Morgan Plantation pipeline
and Colonial Pipeline.

Here's some good info www.colpipe.com    (  www.plantation-ppl.com  more
than you ever wanted to know actually )
As far as automotive gasoline is concerned, except for BP Amoco Premium
"water white",  there is really only generic regular and premium in the
pipelines.  Regular can have up to 5% by volume of miscellaneous product in
it (transmix).

Now as to the EPA attainment areas that's a different story, MTBE can go in
the pipelines, ethanol cannot (it's added at the terminal)
Atlanta, GA and 43 surrounding counties get Atlanta-gasoline only (special
mix made in Baton Rouge refineries).

The web sites above will explain the many types of regular and premium but
you have little choice - what is for sale in your area (MTBE, low sulfur) is
determined by regulations and time of year.

Costco buys the cheapest fuel they can find from anyone.    
Other factors to consider are:  How many times is it filtered before it
gets in your tank?  Texaco filtered their fuel at 5 stages from the pipe
line to your tank.

How careful the delivery truck handles fuel.   If their previous load
was thick road oil and the driver didn't 100 percent clean out the
trailer, that crap goes in your tank. The filter at the station doesn't
always stop foreign liquids.
There was a Shell station that had a dirty delivery truck give them so
much gunk in a load that about 20 cars didn't make it a mile away before
stopping. Shell had to pay for all those that came back to have their
gas tanks removed and fuel systems cleaned.  It took 20 50 gallon drums
to contain the gunk that was in their underground tank.  

A lot of bad gasoline problems are caused by delivery drivers and
trucks.