I've heard a lot about people who have catch-all email accounts and subscribe a different address to each service. So, these people may have a nice idea of who sold or leaked their email addresses based on the spam they are getting. Are you one of these people? Can you name your spammers?
As a side note, I have a friend from not-US who by mistake used a special address only for this country's IRS equivalent (he had something like "unit 12A" instead of just "unit 12"), and he would occasionally get physical spam to that address. I remembered that, then decided to ask this.
They give my address as if it belonged to them. Probably they created addresses like narag33@server and they believe that it's narag@server instead.
So not only I receive all the spam from dubious sites that they suscribed to, but also their legitimate mail from lists and friends.
My namesakes are idiots. But some of the companies responsible of the subscriptions, like Paypal, are assholes. They allow the creation of accounts without verifying the email, then refuse to admit it's their problem and do something about it.