They are a great way to add some personalization into your wedding, help your guests with directions on what to do or where to go and can even double as a thank you card for after the wedding.
You can go as professional or as DIY as you would like.
Sites to obtain a professional look, that I would recommend would be Etsy. Although I have never used this particular seller, a quick search of "wedding signs" will draw up numerous options such as the Rustic Charmer. Remember with Etsy that some prices are negotiable and most items are up for more in depth personalization. Be sure to contact the seller directly and discuss options.
Another option would be DIY. There are many sites that have the templates already there for you, its just a matter of saving the templates and tweaking them to add your own personal touch. Some sites that offer such things are, Martha Stewart Weddings in the template section, Kiss the Groom (who has a kissing tree template) and last but not least, my favorite, The Wedding Chicks (be sure to scour their website for much more free goodies and templates!).
No matter which way you choose remember that this is a great opportunity to have some fun and a great way to add your own personal touch.