How the heck these Spammers get those Unique Email Addresses?

Can anybody please explain to me How the spammers get those Unique site-service-specific emails?

Ok, I understand that it is stupid to post your plain email on your site these days as spambots will find it in no time. It is also wise to buy a Privacy Protection for your website Whois info to prevent mining of your emails, not to mention Never ever submit your email to any publicly accessible service online, etc… etc… all these common sense “regular” measurements to protect your email.

So I do/don’t do any of these things, and I “code” my emails – which means that I use a Specific email for each product or service I buy or subscribe online and Never Ever use that email for anything else!

Yet some of these Unique one-of-a-kind emails suddenly start getting spams. And not just some easy-to-guess emails, but Truly unique specific email - some of them are even from the sites like Yahoo Marketing/Overture, some big directories, Paypal, Domain Names Registrars  (GoDaddy as a prime example),  not to mention services/products I paid for and are from the people I Do Trust they would never sell email of any of their customers as it is simply Not worthy for them.

I even got a spam into one Gmail! Before I finished editing the settings for it! And it is “Google” FGS!

Yes, eventually I do get rid of those emails permanently, but I still would like to know: How do these unique emails get into spammers hands?

Are there so many hackers who specialize on hacking into sites just to get email databases of customers? How come so many ISP/webhosts don’t know about it?

Or is there any other way…?

Could anybody Please explain this to me?
I would be truly thankful!

Dagmar