I'm a man with cerebral palsy in my 40s who wants to see the world. I
walk with crutches but have some limitations with personal
care--bathing, dressing, making food. Organizations i've found on the
web offering travel companions for hire are quite pricey, basically
all expenses paid plus a salary of several hundred dollars a week.
This is both more than i want to pay and more companionship/help than
I need. I want 1-4 hours of help per day, for which i'd happily pay
$15-$20 US per hour. I'm most interested in visiting large cities in Europe and
North America.

Greetings Blaine,
I have been hoping that a G-A Researcher (blue name) can give you a
good answer.  This is just a free comment, hopefully with some
information that you find useful.

First, my respects for your ambition not to let your disability hinder
your desire to travel.

This site may provide you with some interesting information about
Europe and sources:
http://europeforvisitors.com/europe/planner/blp_accessible_general.htm

And this one gives a personal report of experiences with some tips:
http://www.slowtrav.com/europe/disabled_travel.htm

The rest of the site is not directed at people with difficulties, but
some of the ideas about "slow travel" may be appropriate.

I expect that you have browsed through the many sites that can be
found with a search for:  travel disabilities
but here are a couple:
http://www.ricksteves.com/graffiti/archives/disabilities.htm
http://travel.state.gov/travel/tips/brochures/brochures_1228.html
://www.google.com/Top/Society/Disabled/Travel/
http://www.travelguides.org/

I realize that this information is not directed at your specific
question:  how to find the necessary helpful person.

Have you considered group travel for persons with disabilities?
What about just places to stay that can provide the service you need? 
There are also sites where one can look for a travel companion.  They
are aimed at people who want to avoid single room charges, but maybe
someone sympathetic would also be willing to help you  - maybe for a
subsidy of their travel costs.

Maybe none of this is useful to you.  At least, however, a comment
will bring your question back up to where someone else with better
ideas may see it.
I hope so.

Regards, Myoarin