I realize the OP already bought one, but just to keep the discussion going: We buy them at Target. We have an above ground pool. Up to last year my youngest knew to wear one every time the pool ladder was down, whether or not she planned to swim. (That way, should one of the older kids fall off a bike and need stitches or something, I could safely turn away for a few seconds and not have a real crisis on my hands.) The kids also wear them anytime they're on my sister's boat.
As the previous poster mentioned, my youngest had one of the collared ones until she exceeded the weight limit-- that collar really IS important for young kids.
The only thing to really be sure of with a new vest is that the weight parameters match your child, and that it's Coast Guard Certified. If either of those things aren't there, get a different vest. Bright colors are also a plus-- remember: most water looks blue or green, so those colors will NOT stand out! Your child will still be safe, but harder for you to pick out in a crowd.
Each year we check the weight limits and ensure that the vests are in good shape-- no tears or broken straps-- and we're good to go for the summer.
