I obviously spend a fair amount of time fantasizing about D/s. When I do fantasize, I take the parties in whatever my source material is and replace them with me and my Mistress. I don't fantasize about strangers or people I don't have an emotional connection to. While I have a pretty extensive fantasy world in my head, sometimes I look for inspiration from other sources. Tumblr is pretty good for visuals and I have set up a tumblr page for things that inspire me when I see it. The thing about these images is they tend to be a little more on the extreme side due to me just being able to click on the image to repost it vs putting it into context.
Image and video sites are good for inspiration. Literotica is good too, but some of the stories are way too long and convoluted to be good material. There are also sites called webteases. I imagine my Mistress guiding me through one of their stories or using their content for ideas herself. I don't actually follow the websteases, I just read them for new ideas that I wouldn't think of myself. I like that I can search for terms that are in my wheelhouse like feminization, denial, tease, painful. Here are a couple I recently edged to.
Yesterday - http://www.milovana.com/webteases/showtease.php?id=19366
Today - http://www.milovana.com/webteases/showtease.php?id=20594