Wikimedia Commons and Flickr are your friends. Just make damn sure the license on the image allows derivative works, including commercial use, if you're doing the cake professionally (or that the picture has been explicitly released to the Public Domain). And if you're outsourcing the image, you might want to print out the license (or Public Domain statement, as the case may be), and take it along to your supplier, in case there are any questions.
For example, my planned 51st birthday cake is to have images of Squad 51, Engine 51, and Los Angeles County Fire Station 127 (which provided the Station 51 exteriors), from the old Emergency! television series. The Squad and Engine images are on Wikimedia Commons, and have been explicitly placed in the Public Domain by the photographer (with the result that I don't even have to put a photo credit on the cake, although I probably will anyway), and since I pass through Carson almost every Saturday (since I spend the day docenting at the International Printing Museum there), I can drive by Station 127 any time, and snap a quick picture of it.
Also: there is an image editing program called GIMP ("GNU Image Modification Program"), available for WinDoze and Mac, as well as its original native platform (Linux), and while it may be a bit, ahem, "lame" at times (pun intended), it has one big advantage over, say, Photoshop: GIMP is free. Both "free as in speech" and "free as in beer." All it costs you is the time you spend downloading it.
Taking the Station 127 picture, and altering the signage to turn it into Station 51 (which had to be done by a set decorator when they were filming the show) is child's play in GIMP.
Oh, and in the "not wasting the sheet" department, I'd recommend filling any large blank areas with other images you can use on other projects, the way I've done with the two sheets I've had printed.
Edited by hbquikcomjamesl - 1/24/13 at 11:43am