Although rules of etiquette and "netiquette" are much more relaxed in current times than in years past, it's still a good idea to limit cards with off-color or devilish humor to people you know very well.
Halloween Express: Here, you'll find quite the miscellaneous corset collection, from devilish styles to those that look as innocent as a modern corset possibly can.
The following sites either specialize in devilish humor in their e-cards, or have good collections of cards with off-color and naughty humor.
Mary smiled; a devilish twinkle in her eye.
He laughed, his eyes sparkling with a devilish thought.