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!
Good Generator Interlock Company