EBAY!! they have bulk t shirt sales all the time.
Please don't start with t-shirts. Start with woven cotton fabrics and then as you learn to control your machine and handle fabric so it doesn't shrink or pucker, then you can move on to stable knits, and finally to the less stable knits like jersey, which is what most t-shirts are made from. You'll say fewer bad words.
An online store that I like for nice everyday stuff is http://www.fabricmartfabrics.com -- they have frequent sales, the fabrics are well described and photographed, and if you get something you really, really don't think is right, they're good about refunds. I'd suggest something like woven cottons, lightweight to midweight denim or twill, cotton shirting and seersucker and flannel as good starter fabrics. If your local Walmart store has fabrics on bolts, the folks who work in that area often have sewn for years and can help you choose a fabric that isn't difficult to work with and which will work for your intended project. There are also chain fabric stores around but often the folks who work there don't sew, and can accidentally steer you wrong. (One of my students was told that a costume satin was just what she wanted for a book bag -- a) it's hard to sew, b) it's not very abrasion resistant and c) it's not very strong -- and that's just one example!)
At first, you will want some fabric to play with and also you'll want to buy enough fabric and a pattern for your first project. I tend to suggest mostly just buying fabric one project at a time till you start to understand how things go together. You can start building your stash after you get a little practice sewing and find out it's something you enjoy.