Home Depot has the Interlock kits for the newer Square D panels, as does Amazon, (that is for the 200 amp indoor panels).
However, not everyone has those panels. Go figure.
So, if you don’t have a Square D panel, go here: GenInterlock.com, I have used them and they are good.
Of course, contact us if you want someone to take care of all this for you.
We are pretty good!