Looking for a law that says wholesalers cannot sign exclusivity agreements with retailers...something about a ?

The law in question is the Sherman Act Article 1. However, it doesn't quite say that. Antitrust statutes are incredibly simple, leaving it to judges to fill in the gaps through interpretation. Here's what the relevant part of the statute actually says:

Every contract... in restraint of trade... is declared to be illegal...

Exclusivity agreements are not considered per se illegal (in otherwords they aren't always illegal, just sometimes). The are illegal if one of the companies involved has so much market power that they are preventing others from competing fairly. Enforcement is very uneven, as republican administrations (especially the current one) tend not to enforce antitrust laws in favor of the free market.

States also have their own versions of the Sherman act. I don't know California antitrust law specifically, but they are often stricter than the feds. Still, the basic principal is most likely the same.

Note that I am not a lawyer yet (I just passed the New York and Massachusetts bar, not California, but I haven't been admitted to practice yet), and cannot give legal advice. This is an academic answer to an academic question.

If, however, you'd like to hire me to work on antitrust issues, I'm available (not to give legal advice quite yet, but I will be able to do that soon). Send me an e-mail.