Try Buy Sell or Trade Section at this forum and there point to subforum Vouchers - there are a lot of offers of those. Also hosting companies offer them like goddady.
As for Overture (Yahoo), there are promotional links to sign up through, that give free additional credit to your account. PM me for those.
You can find adwords coupon on www.ebay.com. Go to this website then type "adwords coupon" or "adwords voucher" on search field.