Conservationists, millionaires, utopian visionaries--everyone seems to
know how to buy desert islands except me.  In all seriousness, I would
like to know how to find out the prices, locations, and availability
of any unowned but mapped islands, preferably in the Pacific Ocean. 
Where do I find out such information?

Hi Brooklyn--I'm sure your question will get some people to
daydreaming!

You may have a bit of difficulty with Pacific Islands.  One website,
http://www.privateislandsonline.com/oceania.htm , notes that there are
restrictions in that region of the world upon the outright ownership
of an island (there are a few available, though.)

Besides the sites below, you can sometimes find islands up for auction
on eBay!

Private Islands
http://www.privateislandsonline.com/

Vladi Private Islands:
http://www.vladi-private-islands.de/home_e.html

Escape Artist:
http://realestate.escapeartist.com/Properties/Islands/For_Sale/

Searches:
"desert islands for sale"
"islands for sale"
"pacific islands for sale"

Happy hunting!

Thanks for the very thorough answer!  It caused me to spend a whole
workday at the recommended sites, daydreaming away!

You want to buy an Island and you can only pose a $2.00 questions??  haha

I known of an Island for sale but it is not a desert island. I can't
remember the name of it but it is located between Australia and
Tasmania and is about 1km wide by something. I believe the going price
is about $1,400,000 (AUD). It doesn't have an airstrip and the only
access is by boat.
Enjoy.

...but whats a desert island in the pacific as opposed to an island in
the pacific in the same tropical area. Does this mean you want an
island similar to those you see in the millions of desert island jokes
where there is often one castaway male and one beautiful female with
perfect figure and bikini on a tiny patch of sand(desert??) and only
one or two trees.

 Some people say that if you bribe the right person you can lease one
of Indonesia's couple of thousand islands but I would not know where
to start on that quest. Will ask my wife however who is  Indonesian
but currently in Indonesia.

Seeing as you didn't specify a desert island I'd suggest you go to
www.mls.ca and and conduct a search in BC. It's on the Pacific and
there are literally hundreds of islands off the coast. Many are for
sale at given time. Naturally the more remote the cheaper, but the
nicest are amongst the southern Gulf Islands -  which are just north
of the US San Juan islands. Lots of privately owned islands there.

Good luck.