The ISA is to the layers what the CornishX is of the meat birds, in a way.
There are over 30 years of highly selective breeding into this strain. It is not a run-of-the-mill sex link. Developed by I.S.A.-Hubbard conglomerate poultry genetics company. This conglomerate includes Hi-Sex, Bovan, DeKalb and many other divisions.
The only known, licensed seller of ISA's to the public is Townline Hatchery, Zeeland, MI.
I've kept them and thy are absolutely all they are cracked up to be. Utterly incredible, friendly, mannered, early developing, layer of huge, beautiful brown eggs, tightly feathered with awesome feed conversion properties. The ISA holds most of the important brown egg laying records around the world.
They are NOT going to breed true. The F1 hybrid zip is lost relatively quickly in their offspring, as is their look. Here's a photo of some among some BSLs.