People are selling 50 dollar Adword Coupons on the Internet forum but you should be careful to buy them. The coupons are for Professional Adword users to give away to new customers not for them to sell to old once. Google do not want old customers to use them, they want to get new users in to the system.
I know many people that have bought coupons and then used a few dollars and Google blocked them. They can see that you only buy them to promote a site. Google Adword has a system to see when you abuse Adword Coupon. It can affect your Adsense and future business with Adword if you caught ?cheating?.