Etsy is a great source for handmade crafts, and graphics you can print yourself. I recently hosted a shower and I got the inviation design from Pickle Doodle Design. For $15 you get a custom designed JPG file, and I printed them myself on matte photo paper at Sam's Club. If you have a lot of inviations to mail out, it can be a great cost saver over traditional Tiny Prints or other invitation websites.
For an extra touch of cuteness, I used these wrap around address labels:
just print on full sheet sticker paper, cut and wrap!