Overton's has them (see link). $15 & Up. If you're near the lake, I'd check Parafunalia (Gilford & Meredith) and there's alway's Wally World -- they're probably well stocked for the season probably are the cheapest.
The infant vests always have a large head support and a crotch strap.
Happy boating and don't forget the suncreen and an umbrella if you're cruising!
http://www.overtons.com/cgi-bin/overtons/setcat.cgi?2