I want to add power steering to my 1990 sidekick. What do I need to do it? Where can I get it, and how hard is it to do?

Power steering is on my list of upgrades, so I'm interested to find out what it costs and it here are options in the conversion.
I'm guessing it's gonna take a gearbox, pump, pulleys belt,hoses, and brackets .

i got gear from a 94.
the steering box bolted on like my manual one,
get the steering shaft also, cause it's shorter.
the pump bracket and reservoir mounting need to be figured out.
i bought a crank pulley from trail tough to deal with the drive belt.
i have a 1.3L.
big difference- you'll like it.

I want to add power steering to my 1990 sidekick. What do I need to do it? Where can I get it, and how hard is it to do?

You need the PS box with pitman arm, steering shaft from the box to the steering column shaft, all PS lines, pump and pump bracket, reservior, double crank pulley (if you don't have one already) and a new belt.  Since you sidekick is an 8V, you need to make sure the pump bracket is from an 8V....because the offset of the pump from the block is different between an 8V and 16.
You take all the old stuff off and bolt all the new stuff on.  Pretty simple.
There are 2 different mounting configurations for the reservior, depending on what year PS you get.  One will be a simple bolt on for you...the other shouldn't be too hard to get mounted.