I have been experiencing the same problem. This site is great for animals and I've improved immensely with them since using it, but doesn't do much in the way of other things. I tend to go somewhere else now for figure drawing models as well.
I use free-use image site like Pexels, collect 15 or 20 images of the subject I want and then time myself.
I am finding a lot of 'generic plant slideshows' on YouTube. You could always do that and pause it if you need a bit more time for a given image. With regard to backgrounds you could always find a image-centric blogging platform like tumblr or pintrest and collect stuff there for sessions.