Even looking online, you'll find tons of options. For my personal wedding, I looked through Etsy.com to find a designer/design that I really liked. From there, they can design it any which way you'd like and it's very cost-effective.
The most important thing to keep in mind when you start: the size. You'll want to stick to standard sizing. Most wedding invites are A7 (5 inches by 7 inches). It's easy to find envelopes for this size, too.
You'll want to start looking for inspiration. Make a folder on your computer of wedding inspiration, including patterns, textures, colors, etc... Then, just start sketching ideas. Once you've narrowed it down, it's just a matter of getting them printed.