Same here, except my husband installed it himself.
We get a huge savings on our summer hydro bills.

Where does the humidity (water) run into? Any suggestions where to buy humidifier and what size to purchase?

In a stand alone humidifier, the water runs into a holding tank in the unit that needs to be emptied out. If a humidistat is connected to your A/C, it will drain out into the built in drain to the outside as it does when the A/C runs normally. The size of the humidifier is determined by the area (sq.ft.) of the house or room you want to dehumidify.

Humidistats in this part of florida (central) are not really required humidity as well as temperature control is overkill. A good thermostat is all that is needed.
I have lived here for 8 years and i am a retired hvac licensed contractor.
If you lived near either coast where the humidity is higher than it would possibly be needed.
Artco

I have a newer home and the AC system installed has a humidistat setting as well as a temperature setting. it was a Carrier system installed in early 2013 by Sunshine in our Gardenia. The reason I know this is that the tech who came out to check our system showed me all the different type settings I could do with the wall thermostat. Originally I was looking to lower the brightness setting as the light was quite bright. He set the humidistat to 60 per cent. So our AC system already has the humidistat installed.

Just had semi-annual check up
 

by Sun Kool, who installed a new thermostat 3-4 years ago that included the humidistat. Said to set it at 70% humidity and temp of 82 while we are gone.

Said to set it at 60% humidity when we are here and whatever temp we are comfortable with. We use 78 during the day and 76 at night when we are here.