Refer to the thread entitled "How to upgrade outlets and switches"
wherein the OP seeks to replace some incorrectly-installed grounding
receptacles with either non-grounding type or GFCI's. I also had a
similar problem but with only one outlet. For some reason the ground,
normally supplied by the AC cable, was missing even though a grounding
duplex outlet had been installed. IOW using the standard neon light
tester I could not get a circuit between hot (black) and ground on the
outlet, nor between hot and the metal box. I suspect that the cable
was not properly clamped to the box there or at the previous box but
since this is an infrequently used outlet I wasn't planning to pull
the cable to investigate. All other boxes on the same circuit had
ground. The box was far too shallow to install a GFCI so I decided to
put in a non-grounding duplex outlet. Then the problems began.

Naturally I checked the most competent suppliers (Lowes and HD) first.
Lowes had non-grounding duplex receptacles in ivory and white and HD
had them only in white. I really wanted brown to conform to all the
other switches and outlets in the house so, thinking of how much
better the "real" electrical suppliers are according to the
"professionals" on this group, I set out to find what I needed.

First I tried the "real" hardware store where everyone is treated as a
shoplifter. You not only can't browse because such an activity would
take up the expensive conditioned air of the store but everything that
could be physically lifted is kept under lock and key. The owner,
taking time out from complaining about the government with his
layabout cronies deigned to acknowledge my presence with "Yeah?"
typical of such establishments. I ask for a non-grounding duplex
receptacle in brown. He grunts and disappears into the back. Several
hours later--it seemed like that but was probably only ten or so
minutes -- he reappeared and tossed a grounding receptacle on the
counter.

"Er no," say I, "That's a grounding receptacle. The non-grounding type
has only two pins." Mind you, give him his due: He did get the color
right.

"That's all I have," says he, returning to his debate about how the
liberals are ruining the country.

OK, let's move on. My son happens to work for a large commercial
electrician; the type that wires up large buildings and factories.
He's always offering to help me with any electrical problem and he is
on a first name basis with the counter jockeys at (supposedly) the
largest electrical wholesaler in the region. So I explain what I want
and even give him the ivory version I purchased from Lowes.

According to him his encounter with the electrical supplier was far
more cordial than mine was with the hardware guy but still resulted in
a look of amazement at the receptacle and a statement that they
couldn't sell him one because "it's illegal". Although he pointed out
that it's for him personally and not to install on any commercial job
they were adamant.

He continued looking at some of his hardware contacts and in the
meantime I checked that a brown one actually exists. It does. Leviton
list it as part #223.

So off I go to my more residential-related electrical supplier and
after patiently waiting my turn behind half-a-dozen electricians all
discussing how the liberals have ruined the country, the counter
jockey finally gave me the standard, "Yeah?" I proceeded to explain
what I wanted and his reaction was, "You need a time machine."

Much merriment among the electricians at this witticism. I then point
out that it's currently made and is shown in Leviton's catalog as part
#223. Amazingly they had actually heard of Leviton (I'm being
sarcastic). However their reaction was to tell me that I didn't need
the non-grounding type because any receptacle would be automatically
grounded when I screwed it into the box. The "professional"
electricians nod gravely, thinking no doubt that this is a
particularly stupid home-moaner. In most instances they're correct
because all the wiring in the area is and has been since the dawn of
time either conduit, EMT, or AC. However at no time did they suggest
that I test that a ground was actually available at the box. Very
dangerous advice.

I gave up and installed the ivory receptacle from Lowes.