If you are seeking to purchase a toy wagon there are many places. Either in-store, online, or from other people who are reselling their own. So it all depends on your budget, and if you are able to find a good discount or coupons to help out with your purchase.
Online there are many websites that sell wagons for kids. Amazon is a great place to find affordable new, high-quality wagons. Specific brand names websites such as Radioflyer.com is going to have Radio Flyer wagons for sale at full price. You also can find wooden or plastic wagons through online shopping sites like Target.com, Fredmeyers.com, and Walmart.com. In addition, they are often carried in these same brick-and-mortar stores. Google is a great way to do an initial search for wagons you might be interested in buying.