Mine were complete swap outs, the original headrests are sat in the garage.
I got mine from eBay, just have to watch out for HK or China cheap ones.
My inspiration came from a US site called OnFair.
You should be able to pick up a pair off of eBay without any problems, just watch out for the sellers feedback and customer service as this tends to be the only real difference between what is sold.
I went for 9" HD screens but you can get lesser quality.
Both the boys love them, they can watch the same thing or different with the way I have set them up.
You have to watch out for the fact that they will want them on for all trips for the first month or so, which can be challenging.
Once zipped up it's hard to tell they are there.
Best of luck on your search, they are well worth the effort if you have kids. :-)