I was recently talking to a fiend who become an entrepreneur - starting his own consulting business and explaining the importance of having custom printed stationaries including business cards. Using blank generic stationeries can give the impression that you are not serious about your business - and some may question your professionalism.
If you are thinking of getting custom printed letterheads, I would recommend Vistprint, one of the best and cheapest places on the web to get customized stationaries. The prices start at only Starting at $19.99.
You can choose a design and customize it by uploading your logo or you can upload your own designs. If you want it really special, they can even create a professional design for you according to your instructions. Once you have created and ordered them, you can have it delivered to your doorstep as fast as 3 days.
You have the choice of getting matching envelopes when you are about to checkout. Vistaprint uses full-color printing on 70lb. paper stock and so far I am quite satisfied with their service.