There was a time where Google was sending them out by mail (they may still be doing this). I've also heard of people signing up and when Google gives them a call (Google usually gives you a phone call to see how everything is going with your new account) they strike a deal with the agent over the phone.
I get sent Adwords coupons from Bluehost 1-2 times a year. They also come stuck inside FastCompany and Inc Magazine quite a bit.
Magazines like .Net had them every month while I read it.
in startup deal sites like rewardli.
I have used a couple of hosting services, Blue Host and (I think) Monster Host, which offered deals of this sort, as well as other deals. I also have an adsense account and periodically get offers in the mail.
Yes! forgot about that one. Hosting services practically hand them out.