I would recommend you a simple three-fold approach: 1. Think of an existing Web application that you like a lot 2. Think of a feature it's either missing or s

I would recommend you a simple three-fold approach: 1. Think of an existing Web application that you like a lot 2. Think of a feature it's either missing or something that can be simplified in the app 3. Build a stripped-down version of the app in your spare time as a side project. Launch, gather feedback & analytics, improve, repeat. This approach will solve your concern with finding the idea and executing it with limited resources.