Ebay will cut the price somewhat. The only other thing is to find a shoe store's coupon for 20% off.
They're kind of like Tom's though: you're just not going to find many deals because they don't need to make deals to sell 'em.
However, luckily, one pair of Sperry's will last about 3 years unless the child in question outgrows them, so they can be an investment.
And if they are outgrown, you can always sell 'em on E-bay yourself and get most of the money back.