The locations of all 20 bobbleheads can be found here. I have mentioned above in certain achievements/quests the two that can be missed. The other 18 can be obtained at any point in the game, unless you blow up Megaton which I also mentioned above.
Note for PC Users: Again, to save you a lot of time and searching, use the command "player.placeatme 38832 X" to spawn a number of bobbleheads on the ground around you, X being the amount. This will spawn the exact same one but it doesn't matter for the achievement. You don't have to collect all 20 different ones, just 20 total.