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.
Buying/Finding A Design: If you don't have design experience, and you're not fond of pushing images and text around on pages (aka, does the idea of designing a flyer for a Bake Sale or some other event excite you, or does it make you hide under your desk?), you're going to need to find someone to design your DIY wedding invitations for you. Fear not! Thanks to the Internet (and Etsy, bless), there are tons of places to get an invitation design. Some are even our sponsors! Here's a short list of places to check out.