I sometimes go months without rendering anything and then sometimes I have so many ideas that I can't get them all done.

As previously mentioned, going for a walk in the park is a good idea - you might see something you want to work into a render.  Sometimes I go into a cafe and see a painting on the wall and decide to do my own version of it in Poser or Vue.  I was on a bus in Italy a couple of years ago and I saw a young girl sitting outside her house at the side of the road reading and it gave me an idea for a render.

If getting out and about is difficult, then take a look at online galleries.  I saw one of Margaret Keane's pictures online a while back.  I typed her name into Google and looked at her images - straight away the ideas come flooding.

I also once saw a scene of a city on one of the Star Trek films and thought - I'm going to do a version of that.

I would never try to copy an image, but looking at what others have done is good for inspiration, and sometimes it forces us to have to learn new techniques to get a desired result.

i7 5960X, 32GB RAM, GTX 1080Ti, GTX 980 Ti, Windows 10 Professional.  Running Daz Studio 4.11, Poser 11, Vue Inf 7, Photoshop CS4