Double-sided tape is the best ingredient for making cards. Every once in a while I grab a stack of magazines, cardstock, old cards, a nice pen, a pair of scissors and some double-sided tape to recraft a card for a friend or family member.
Cards at stores can be lovely but I find they often don't quite capture what I want to say, or those that do cost $8, which seems insane to me (to pay that much for just paper). So when I can, I enjoy making cards for birthdays or weddings or house-warmings or baby arrivals or whatever.
There is a craft store, the Urban Source, close to my home that sells bits and pieces of recycled and new crafting pieces I often stop in for augmentations to my magazine pile or for an extra roll of double-sided tape. This time I found some pretty little bird cutout stickers that were actually mirrors. Very cool.
A note on envelopes: The urban source has a great selection of strong coloured good quality envelopes. I have also found that just about any store selling cards has a good backstock of extra envelopes and will either give or sell cheap (5 or 10/$1) these extras.