A friend of mine tipped me off to the website http://www.cheapestees.com/ where they have some great prices on polo shirts. If you choose "Youth, Infant, Toddler t-shirts" there are several different options for polo shirts.
The "Gildan Ultra Blend Youth 50/50 Jersey Polo shirt is only $3.94 right now and the white one is only $2.94.
They offer free shipping on orders over $69. If you don't need to spend that much, maybe coordinate with another parent or two to get up to $69 to save on shipping.
Click on "Can't find it" and answer their very short survey. I did and got a coupon code for $10 off a $50 order.