No heat on passengers side of van

Remove the lower trim panel on the drivers side. Look up onto the - passenger - side of the HVAC box and the actuator is screwed on with three screws above the air duct. The drivers side unit will be the first one you see the passenger control is on the opposite side but in the same general location on the box.

Looks like the part is a dealer item, not surprising with Kia.
Just finding the correct information can be difficult as Kia and Hyundai switched to paid service manual information in 2014, but they do not have really good information to begin with. There are some items they give full data on then others where you get virtually nothing, HVAC falls into the second category.
Just as a point changing the heater core, They show a book time of four hours, but as for instructions as to what to remove to get to it, there is nothing.

From what the factory info shows you can get to that actuator from the drivers side, Not sure I believe it but that is what the book says. Like I said, Kia tends to be somewhat difficult when it comes to service information on some models.

I have seen the 2007 video on replacing the blend actuator and that heater case looks a lot like what they show for the 2009. I just went and compared the factory information on both the 2007 and 2009. It shows the same pictures and removal information for both. Which must be wrong as this video shows the removal in an 2007 and the instructions do not match how to get there.

These are the factory instructions for removing the passenger side actuator on both years.

Disconnect the negative (-) battery terminal.
Remove the driver's crush pad lower panel.
Disconnect the temperature control actuator connector (A) after removing the air duct.
Loosen the mounting screw and then remove the temperature control actuator (B).

This shows how to actually get there.
https://www.youtube.com/watch?v=JZ9FtRXtClo

Personally I would probably open the glove box and see if you can see the actuator like in the video and go from there.

As to where to get the part, It looks like it is dealer only, I do not find a part number even in the factory book!