I would like to buy a real USPS postal box.  I am not talking about a
residential mailbox.  I am talking about the big blue boxes which dot
the corners of our streets.  Alternatively, I would be interested in
buying a green "mail relay" box (similar to the blue ones, but without
a slot for accepting mail).  I would prefer the mailbox be used, but
new is okay.

Go to mailboxes.com and look under commercial mail boxes where you can
scroll down and click on collection boxes. You can then scroll down
and I think you will find what you are looking for. Good luck

Well done.  I spent some time searching but couldn't come up with
those terms.  Of course, these aren't blue or green, but the paint
job, even professionally done, can't be too expensive.

Thanks, but (and I am sorry if I sound picky here...) I am looking for
an original mailbox.  I would love one with graffiti and salt burns
and all that good stuff...

Dolphindave tells where you can get a brand new mail box.
What about chatting up your local Postmaster and offering him a free
replacement for your choice (his eyesore) of an old one?
(I won't expand on my first idea about a van and a dark night...)

The Postmaster is a good idea, but how would I get in touch with him
or her?  If you can tell me how and the idea works, seems to me the
$50 is yours...

On the other hand, if you should choose to go the van route then I
take no responsibility for your actions... but I won't ask any
questions when you offer to sell me an "extra" one that you have
laying around ;)

1-800-ASK-USPS - call them and ask for your local station post office
phone number.  They'll be happy to tell you.

Or http://www.mapsonus.com/db/USPS/
Enter street address, etc.,
Click Submit

Should show you a list of local Post Offices

msg43-ga,
hope you're still reading, if you go my route, offering a replacement,
be sure to have them give the box unlocked.  I expect that they can't
give you a key, but then you could replace the lock so that you have
use of the space.
Good luck, myoarin

From my search, it appears that the official supplier of the USPS is
Postal Products Unlimited, Inc. Here is a box "made to the same
measurements as the USPS collection box, but for private use only.":
http://www.postalproducts.com/ProductSearch.asp?SelStart1=N1004687
It comes in gray, but you can have it custom colored any color by the
company, except in... blue.
This leads me to believe that there may be some federal law governing
the sale and/or reproduction of government mail receptacles (and other
products?).

However, on the USPS site, this is the answer to one of their FAQs:
" Where can I attend postal auctions (sales of postal equipment)?  
     
     Auctions and sales to dispose of postal equipment are also
conducted locally to foster community involvement and reduce handling
and transportation costs. These sales are announced in local
newspapers and by notices posted on lobby bulletin boards at the Post
Offices where the sales will be held. US Postal Service Purchasing
Service Centers maintain lists of those interested in making purchases
in their geographic areas."
You can get added to the list by clicking on the link and writing to
the nearest address posted.
http://hdusps.esecurecare.net/cgi-bin/hdusps.cfg/php/enduser/std_adp.php?p_faqid=1184&p_created=1052869690

I know you can get trucks; I don't know if they keep the original
paint. I also don't know if you can get the boxes. Also, if you
contact your local post office (or maybe just the big branch in your
city would be most effective), you can find out about auctions in the
area.

And it seems a term good to use in future searches is the "industry
term": "collection bin." That's what I ended up getting most effective
results with.