Not sure what you mean by a pool float, but you can buy baby life jackets that will fit a newborn at sporting goods stores. We bought one to use on my parent's boat.
We bought one at Target the other day for $14.99. It's by a company called Banzai and it's an inflatable puffer fish. It says for ages 2+, but we don't do it for very long (10ish mins max).
Re: anyone have a pool float that fits a 0-3 month old...