I need a heat-to-electric energy converter for a project, are those commercially available?

I need one for a specific project, and don't want to hand-make it. It needs to successfully convert heat energy into electric energy with minimal energy loss. I would like to know if I can buy one. I live somewhere in the Midwest of the USA, and I have a certain distance limit I have to follow. I also have a certain budget, so it can't be too expensive. Where and how can I obtain those, if possible?