I am thinking of buying a tortoise. Before I do I would like to be more informed on what kind of foods they eat.

Most countries have a health and safety policy when it comes to food. These standards are vigorously met, or the product is destroyed or declared un-fit for human consumption.
If you buy food products from shops and other places they have to meet the standards. Products sold off a van or online might break these laws. The import of such food (not fit for human consumption) is monitored by trading standards and disposed of.
If you suspect that something you have purchased is not up to standard you should report it to your local trading standards or food and safety body.
What you buy in a Supermarket is generally above standard at least in the developed countries. In fact much edible food is thrown away just because it is bruised or damaged, than dangerous to eat.
Apart from a few stories were someone has done something dodgy, I think most of these stories are simply scare stories.

A dog can go without food for a few days and then will eat when it is hungry. Just stop giving your dog your food and at its normal feeding time give it a bowl of its food. If it does not eat it, take the food away. Then 24 hours later put the bowl of food out for the dog. If it has not eaten it within ten minutes take it away and then 24 hours later put the bowl of food out for the dog again. By now the dog should be happy to eat what it is given.

"Beetles eat all kinds of food. Most are specialists in few kinds, but some, like ground beetles, eat lots of things. Most beetles eat plant parts, either leaves or seeds or fruit or wood. Many are predators on other small animals. Some eat fungus, and there are a bunch of species that eat dung. Sometimes the larvae eat different foods than the adults do."

http://www.biokids.umich.edu/critters/Coleoptera/

That depends on the type of tortoise. A tortoise five centimeters in length obviously requires a different environment from a tortoise two meters in length.
Take a look at https://en.wikipedia.org/wiki/Tortoise

Canned fruits and meats are excellent to keep for emergency. Also, vegetables that last for a long period of time should be bought for emergencies. These vegetables include potatoes, squash, apples, carrots, and more. Finally, powdered milk and ready to eat cereals are good as well.

Herbivore eating amphibians like to eat food items like romaine lettuce, bananas, chopped squash and chopped collard greens.

That all depends on what of reptile/amphibian it is - carnivore, herbivore or omnivore. Most snakes and lizards are carnivores, green iguanas and some tortoises are herbivores and bearded dragons and most aquatic turtles are omnivores.

There aresome foods that we humans eat that dogs can eat as well. Such food consists ofyogurt, flax seeds, apples, oatmeal, bananas, and more. For more information,check out these websites: http://www.pawposse.com/what-human-food-can-dogs-eat/blog-297/and http://www.pawnation.com/2010/07/28/10-human-foods-dogs-can-eat