What kind of invitation do you want to make?  First search for DIY wedding invitations online and see what style you like.  Come up with a design you want. Then decide on whether you want it professionally printed or use your own printer.  (Look for local printer shops to get an idea of prices- close to me a Chinese printer can do 100 invitation text in thermography (letter ink raised up from page) for $60.  Then, look at paper at craft or scrapbook stores to see what you like, you can order bulk online.  Then decide on text and font. 

Try looking at  www.mygatsby.com, http://www.renaissancewritings.com/ for ideas.

Buy bulk cardstock from Michael's for cheaper.