Where can I get white plain shirts to design myself like writing on it?
just go to your local Walmart, in the men's or boy's section and look for Hanes or fruit of the looms plain or pocket white tees in your size.
From the craft section of Wal-Mart, or any craft store, I suppose.
Most normal stores have plain white t's, You should probably check the nearest Walmart
$1.95 each here:http://www.jiffyshirts.com/jerzees-363.html
Hobby lobby :)
Just about anywhere, I've even seen them in some grocery stores.