go to gaming then find lost planet.
to me this was the best way for finding the target markers, as it shows a pic of everyone of them and how to get them. Actually this was the only site I used.
credit to dmbledore for making an excellent guide
oh as someone pointed out you have to collect all target marks in every mission across all difficulties. So that means beating it on easy all coins, normal all coins, hard all coins, and extreme all coins. but you showed know that. the guide above will show you each and every spot for the coins. from easy to extreme.
also If you go through a mission and forget a target mark you can always go back and get that one you don't have to get the same ones again. Most likely you will have to replay some extreme missions. since some levels are so vast you won't be able to get all coins then beat the level. mostly missions 3, 8, 9, 11.