Most newsagents that do Paypoint will do them. Just walk in and ask and they will print one off for you,

If you google or go on to the paysafecard site it will tell you where your local shops that sell them are
You can only get them in vouchers of 10 20. 25 etc no 5pound vouchers

My local co op do paysafe and so do spar and most garages. generally anywhere that they offer paypoint payments like topping up electric and gas preypayment cards

I would recommend that you set up an account on paysafe then when you buy say  20 vouchers and have £5 left over on one of the  vouchers you can then put the voucher number in the account and the amount left over will be on the account 
So if you buy £20 to pay for a £12 Viva Advert you have £8 left on the voucher. You can either keep the voucher and when you put up. Another add use the voucher for £8 and then buy another voucher for the difference

Because of the price of viva st you will Always have an odd amount left over

If you put the amount on the paysafe account. When you log into viva st you can pay using your  paysafe account with the amount you have in it

If you lose the voucher you lose the cash unless you screen shot the voucher number