you can get gift vouchers from any of the suppliers like sallys, salon services, sassi.
I do mine on vistaprint.
I use a notelet template and type Gift Voucher on the front cover
inside I pay extra for it to be in colour, I write a message and further down I type in my contact details.
when I get them, I print a smaller picture on the plain envelopes, or you can get vistaprint to do it for you at an extra cost.
www.vistaprint.co.uk/specialfree1
I also make my own using plain cards from Tescos, you got 50 in a pack with matching envelope, and can get them in white or cream.
I go into word, put in the dimensions and then choose a picture to go on the front, the best thing about this is you can personalise them, put more of your own details on it, etc.