Flat unfolded cards and traditional card blanks are great if you’re on a budget or don’t have huge amounts of information to include. Wallets are great for including lots of extra information inserts, RSVP cards as well as the main invitations themselves. There are also in_between options that give you the freedom of a wallet on the budget of a traditional card.
If you're buying your own paper, you can find a good selection at local office shops when it comes to card stocks and glossy photo paper. However, if you're looking for linens or specialty papers, you may have to go online. Always get a sample before buying a huge stack of paper, and do a test print, to make sure that whoever or where ever you're printing from can actually use the paper you've selected with your design and medium. Here are a few other places to buy paper from.