There are many places to get both iron on transfers and tshirts.
You can buy the transfers at places like Walmart, Target, Joann's, Michaels, Staples, Office Max, etc... I believe all of these places have them. Be sure to know the difference between light and dark transfers before you buy them. I have tried lots of different brands of transfers and have now found the best dark transfers on the market. See my webpage (link below) for more information.
For tshirts, it really depends on how many you are wanting to buy. If you are buying quite a few and you would like matching tshirts - jiffyshirts.com is incredible. They have many brands, colors and sizes and they ship very quickly. If you are only buying a few, I have bought from Target, Walmart, Michaels and Joann's.