Nintendo eshop vouchers - Where do you purchase them reliably?

Jo4

I have been asked to get Nintendo eshop vouchers but when I tried to purchase them online I was told I needed an account.  Can someone tell me where I can reliably purchase these vouchers?  Also. is it true that you can only pay for items using one voucher?  The voucher will be a Christmas gift and since it is the only present the recipient wants 4 people I know are giving the same present so that is why I was wondering if we all need to purchase one voucher between us?!?!

What values can the vouchers be purchased in?

I would prefer to purchase the vouchers online if possible.