Most supermarkets have a great deal of offers online, and generally it is cheaper to shop online than going into the store yourself. So I would suggest you try these out.
Alternatively there are plenty of voucher or discount sites out there that promote great deals, offers and have money-offer vouchers if you look in the right place! Having used these type of sites frequently I can confidently say that moneysupermarket.com is one of the best.
The great thing about moneysupermarket is that they don't just cover food and drink vouchers, but things such as insurance, travel, mobile phones and even energy bills.