I would like to purchase and install a functional, quality firepole in
my house, to go between the first and second floors. I don't want a
playground-quality pole, and it must be able to support a 250 pound
person. Can you find a vendor of the item and installation
instructions, or find a contractor who can perform the work? I am in
Washington state (US).

It won't be cheap:

http://web.recordnet.com/daily/columnists/hood/1lodi070702.shtml

"The price of new poles seven years ago was roughly $10,000, he said."

and who knows if you can do it "officially", that is get it past
building permits/inspectors...

"Washington state's legislature banned poles from new stations in 1997..."

That is, if they won't allow them in fire stations, will they allow
them in a private residence?

All of the above said, you probably should get a structural engineer
to do a study and/or draw up construction plans that you could in turn
take to most general contractors for the construction.  Add a comment
with your general local in Washington and someone can come back with
a list of structural engineers in your area.

Lets not go mad here.  I'm a Structural Engineer and I can tell you
that 250lbs is a very light load in structural engineering terms.  The
first thing you have to address is whats with the firepole.  If you're
250lbs are you a wise man to be sliding down firepoles?  Maybe its
some sort of fetish thing with you but my advise to you , before you
spend money on your pole and installation go see a psychologist.