My stepfather is 82 years old and his hip has been dislocating. His
doctor says he needs to 'sit higher'. We have tried with various
cushions but they just get squished down. Is there a kind of foan
rubber that holds its shape better than regular cheap foan cushions?
We could use about 4-6 inches of elevation. Thanks in advance for your
help here.
-ethan

Have you tried a T-foam cushion? They are not inexpensive, but they
offer good support and comfort.

http://www.allegromedical.com/cushions_covers_backs/cushions_drop_seats/t_foam_cushion_16x16x4_medium_t41.P199447

I won't ask why a foam cushion can cost $80, mainly because I would
suggest an even more expensive solution:  a new chair for the elderly.
 My father was pleased with his and my mother-in-law even more so.

It doesn't have to be something like this:
http://products.chairs.ws/8261

I was thinking more along this line:
http://www.highseatchairs.co.uk/pages/welcome.asp

but, of course, from a US producers, if that is where you are.
Maybe this:
http://www.craigslist.org/pen/spo/96000263.html

There must be other US products similar to the English ones.
Admittedly, this is a much more costly solution, but one that may
eventually have to be considered.

That's a point - where are you  located?

Also, have a look at the lounge rooms in your local retirement
villages - chance are they are furnished with high seated chairs -
they range from tacky plastic to elegant furniture.

Is a hip replacement out of the question? Mine only took 1.5 hours
through a tiny incision, but I am not quite 60, and my insurance
covered most of it. For his age group its a matter of general health
and motivation, if cost isn't a factor.

The simplest way is probably to use chair raisers under the legs. This
is the sort of thing, but they may be available free from your local
social work occupational health department.
http://www.bupa-shop.co.uk/index.cfm?articleid=1251
http://store.devinemedical.us/stcora.html
http://www.westons.com/acatalog/Online_Catalogue_Chair_Raisers_117.html

Owain