Hi Katy--- You want to do cabinet refacing and it's a great alternative to ripping those old cabinets out. You can hire people to do refacing for you but there is no reason why any homeowner who wants to can't do a professional quality job of it themselves. In answer to your question, yes, there are places where you can buy just doors. Many of these places will deal only with professionals but a few will sell to homeowners. One such company is Quality Doors which markets to homeowners through Home Depot. Another possibility is to go to www.reface.com there you will find a company that sells refacing products. There are other internet providers. If you want to deal locally (and that is usually a good idea) there should be cabinet shops near you that would be glad to provide doors. many of these shops don't make the doors, they buy them from a door supplier.
In any event, while you are putting those doors on and changing the style of your kitchen, you will want to also cover the cabinet faces and sides with a matching wood. If you put a crown molding around the top of the cabinets, the new kitchen will look like a million bucks. How about pull out shelves in the lower cabinets? You can get all the how-to details about cabinet refacing from a book I've recently written on the subject (the only book of it's kind on the market). It is published by The Taunton Press, which also publishes Fine Homebuilding magazine. If you go to www.taunton.com you can find your way to their bookstore and read about the book. It's called "Refacing Cabinets: Making an Old Kitchen New". In fact you can read the entire introduction which explains about refacing and what the book will teach you.
If there are any people out there who are interested in getting into the business of cabinet refacing, I publish a newsletter called Professional Kitchen Craftsman which is geared primarily for the cabinet refacer. There are now PKC readers in all 50 states. E-Mail me for details about subscribing if you are interested.