I have filed my Federal taxes and obtained ITIN for my wife and kid.
I believe, I still need to file my State Tax for Pennsylvania.
What form am I supposed to file ?
Most of the sites (turbotax , www.taxactline.com etc) allow you to file both Federal and State tax together.

Since I have already filed Federal tax and have the ITIN for my wife and kid, can I file just the state tax online ?
If yes, could you please let me know the form name and or/site.
If I cannot file online for some reason ,where (website) can I get the forms for State tax?

Try the Pennsylvania state government website.

Many states have an online fling capability embedded on the state tax authority's website.

At the very least, you can download the required forms and sintructions from the PA state tax authority website.