Can you help me find a shirt for swimming?

I'm starting swimming tomorrow and I'm like really skinny. I asked my pe teacher if I could wear a shirt and he said as long as its not my pe shirt than its fine. I wondered where I could buy a shirt for swimming or a shirt thats good for swimming. All of the shirts that I have would weigh me down but I've seen people with shirts with no sleeves that look pretty light. Any idea where I can buy one of those? Or do you think that any sleeveless shirt will be okay to use? If you could send me a link that would be great. Best answer will get 10 points.