Hi, I would suggest applying for a micro-loan, depending on the amount you need, and what the useful idea is. There are lots of groups giving out loans for new projects.
The other thing I would try is business angels. Find a group near you, get to know them and what the investors are looking for, and pitch your idea to them.
Do you have a business plan for your idea? If not I would put one together showcasing the opportunity and potential profits. There are some great templates online for free and writing a business plan forces you to think through some of the key points and decisions before you go too far with an idea.
Best of luck with your idea. Perhaps you can post a hub detailing your experiences?!