Yes, they are Mardi Gras beads. I'm not sure which groups give them away, but they don't hand out that many.
The foam balls shoot out of the big floats in Block Party Bash at the end of the show. Just wait by one of the tall floats and you should be able to grab one or two. The characters also toss a few into the crowd, but don't count on getting any that way. When we have nothing better to do, my wife and I have followed the parade around to catch more balls. When I get extras, I usually give them to kids who didn't manage to catch one.
They aren't quite the size of tennis balls. They're closer to golf balls. Nerf is printed on one side and Block Party Bash is on the other side. Nothing that special. They come in red, green, blue, yellow, and maybe some other colors.
Lloyd Lee-Lim