Craft Creations have loads of card and paper to choose from, and they make the cards with a line in them which makes folding them virtually foolproof... Lots of different sizes too. I got ours from there and they were great, quick delivery.
As for tips, definitely choose a simple design - they will take you hours anyway! And double sided tape can be v handy for sticking things, as well as almost invisible even through thin paper. Get good quality envelopes cos they'll finish off your invites nicely. And make sure you've got space to put them once you've made them all!
Jo