The WWE store, Amazon, Walmart and Kmart all stock children's WWE belts, as of October 2015. Amazon appears to have the widest range and lower prices. Belts are ordered online from the WWE store and Amazon, while Walmart and Kmart may stock the belts in-store.
The WWE store has a boy's Heavyweight Championship belt and a girl's Divas Championship belt in stock. Amazon carries the same two belts, along with several extra options: the United States Championship, Intercontinental Championship and Tag Team Championship belts are also available in children's sizes. Amazon's prices appear to be slightly lower.
Both Amazon and the WWE store carry other official memorabilia along with the belts, much of which is aimed at children.
Walmart's website shows that it carries the WWE Championship belt, while Kmart's site shows that the Intercontinental Championship belt is available. Both of these stores offer shipping, although the belts may also be available in-store. The prices at Walmart and Kmart appear to be comparable with Amazon.
For new belts, along with those from previous years, eBay has many listings, as of October 2015. Prices there may vary as some of the older belts may be more rare and attract more bids.