Gustav Klimt appeared to require lovely young women lounging about his studio to inspire his work. Many old masters were created as a result of the assistance of a muse. A walk in the countryside may be all it takes to fire up your desire to attack the canvas with brushes or palette knife.
An effective combination of colours put together in an image on television, introductory graphics or in a photograph could drive you to sit down and paint. A poem or some beautiful prose may set you in motion.
There is something to be said for browsing through magazines or old photographs. You may not want to replicate them, but to create a composition from something or someone in them.
The way light falls on water, shapes formed through branches of trees could be the beginning of a piece of work the result of which may surprise you.
Travel brochures can be an endless source of landscapes, but it is up to you to bring them to life, not to paint the sky so blue it is absolutely unbelievable and to add or remove figures where necessary.
Wind and shadows change the vista from the same window from which we view our world. Rain makes for interest amongst the shrubbery and brings out umbrellas that are at your command for colour and size.
Dont forget to take your sketch book with you when you go out and take down enough information for your painting, especially the light source.
As long as the inspiration to continue to paint keeps coming, and if you visit many of the artists' sites on the web I am sure it will, you will never be at a loss for subject matter. Just make the work your own individual product, copies are good for practice but you need your own finished pieces, too.