Car seat bases...

So my sons almost 14 months and we're planning to get him one of the Graco car seats after he grows out of his infant car seat, Id like to get him either the Graco Milestone or 4-ever car seat... but if we need to get an extra base for another car... where would we get them at?? It doesn't say that they're compatible with the click connect bases and it looks like they have their own base