Ok, so I had a ton of trouble finding the last few that I was missing, because the numbers that correspond to most of the guides out there are wrong (Achievement hunter is the one I used, which uses the wrong numbers).
One of the other solutions used this link/guide, which I found very helpful, and got me to 151 out of 159:
http://achievementhunter.com/games/guides/mafia2/index.phpThis guide lets you click on each neighbourhood and breaks it down very well. I figured out a system and went neighbourhood by neighbourhood. Of course, there were some that I had thought I already found during my playthrough and I moved on. At the end, you will undoubtedly find a few that you missed.
Go to the main screen and click on extras/collectibles and then wanted posters and make a note of the ones you are missing.
*Imprtant note: these wanted poster numbers from the actual game will not correspond to the first guide that I linked you to above, but don't close the first guide, you may need to work off of both, as the solution videos on youtube are all made to correspond with the first guide (achievement hunter) which is incorrect*
Now that you have only a small number of the wanted posters you haven't located yet, open this link up:
Take your list of unlocated wanted posters and find their location on the Wikicheats map. Now you will know where they are exactly.
Having said that... If you get there and just can not find them, you will have no luck searching youtube looking for wanted poster #__ if you are using the actual number of the poster from the game.
For example, if you are missing #63 (the one on the house), you will find it on WikiCheats, then look at the same location on AchievementHunter and find that they use #62. So you will search for a solution to wanted map #62, even though you are actually looking for #63 - confusing... but there is no other way that I could find.
Since you know where the poster should be, because you found it's location on the WikiCheats map, this is where you may have to refer back to the first guide and look at the location you know the wanted poster should be in order to find the number used by the youtube solutions and achievment hunter.
I found a quick google search with: "Where is wanted poster __" always brought up the right video.
Anyway... I hope this helps people. I was pulling my hair out.