You can get it from any chemical store, or online such as UnitedNuclear.com, but cesium is very expensive it costs more than a hundred dollars for a mere few grams.
To find elements that are chemically similar, you don't need to look further than the related elements in the same family (columns). The periodic table is formed so elements in the rows have similar...
Because they are extremely reactive to any moisture in the air. See the Related Questions about the reactivity of the alkali metals (rubidium and cesium are the most reactive of the alkali metals).