M.M.
You can pick one up at Wal-Mart, Bass Pro-Shop, Calbelas, just depends on how much money you want to spend the same thing. What you need to look for it that is is COAST GUARD APPROVED! It will say that on the inside of chest piece on the warning label there. Next for a child that small make sure it has a 'collar' on the back of vest, so when the child lays back it will support the head and keep the head above water. They are sold by size & weight, this is one item you do not want your child to grow into..so make sure it fits. Put it on the child and buckle all the buckles, make sure the sides of the vest are touching not overlaping, if they are overlapping or not touching this is an incorrect fit, buckle the strap that goes from the back of the vest between the legs to the front of the vest,make sure this it tight as well. Dont worry about it hurting your child, it is not going to be like a thong or anything. Once you have it on, to ensure a proper fit and that the vest wont come off of the child, put the child in front of your so you are looking at his back, grab the shoulders of the vest on the shoulders, holding the vest with the top of your hand on the top of the childs shoulder. Then pick your child up by the vest, try it also with his arms up in the air this will show you if the vest is too big. If your child falls out of it then you know it is too big. This was always the favorite part for my son when we had to get him a new 'swim jacket'. Most of the kids vest come in kid designs on them to make them more appealing.
Good luck! If you go to one of the speciality shops, Cabelas or Bass Pro, they might be able to give you more tips, but this is what we learned from our lifeguards.
Good luck.