Splined disconnect individual parts are ONLY available from GM suppliers, and there are two typically used: parts4chevys.com (alternate name for gmpartsshop.com) and gmpartsdirect.com. After years of buying parts, I've gone exclusively to parts4chevys.com due to easier phone access, skilled staff, and more honest pricing. They have a decent markup over wholesale, and fair shipping charges. Gmpartsdirect.com appears to have a tactic of suckering in the unwary by offering flat wholesale prices for the parts, and making it up with inflated shipping. If you look at shipped costs, they're about the same, with my experience being gmpartsdirect.com often a bit higher because of my distance from them. But I object morally to their sneaky way to get business, so I boycott them for the sneakiness issue.
For the splined disconnect specifically, the best price people have seen on the sole aftermarket supplier of the assembly, who doesn't sell individual parts, is Amazon at $400. Search for "ATP 111001 Front Axle Disconnect"
I agree with the other recommendation for rockauto.com for parts with multiple suppliers including aftermarket.