where can i find some good resources on flat tires?

last week, i got a flat from my 27x1-1/4 rear tube. it was after I finished my first paper route using my cargo bike trailer, so I assumed that the flat was caused by the amount of work the tire had to do, so I switched my delivery to a mtb.

Now, a week later, I got another flat. The new tube is a 27x1-1/8. The tire was fine all week and before I left home today. I'm not sure when the tubes gave out, but I discovered that they were completely flat after I finished playing tennis.

I'm going to do the bubble test in a moment, but I suspect the cause is not a hole in the tube.

Lastly, I'm looking for some durable tubes that are also inexpensive. Where can I get them? Amazon? bike shop?