A display ad can be placed in most any magazine, trade journal, convention program book, even your local high school events calendar.
With the advent of the internet the adage "the more things change the more they stay the same" is proven all the truer. Having a website is not a magical prescription for sales. No more than having a shop on a street "Somewhere USA" is a magical prescription for sales. Whether brick and mortar or cyberspace you still need to bring the customer through the door.
Display ads are one way to bring in customers. Just use the same common sense you would if advertising a brick and mortar.