There is no such thing as a flat-proof tube. There are some that are flat-resistant, usually by making the tube from thicker rubber, and there are some that claim to be able to self-repair small leaks, but I've had flats with both.

If you want the most flat protection you can get, start with a kevlar-lined tire like an Armadillo or a Marathon Plus, add in a tire liner, and then put in the thick tubes. It'll weigh a ton, but it will be pretty flat-resistant.

Personally, I use kevlar-lined tires with tire liners, but just regular tubes. I haven't had a flat since I switched to this configuration and I pass through an industrial area twice a day.