I dont know about you, but i can never be bothered finding all the collectables myself. so im gonna hand you over to the guys at IGN and Gamesradar who've made some lovely guides showing you where they all are.
hope it helps:http://www.gamesradar.com/assassins-creed-revelations-data-f...http://uk.ign.com/wikis/assassins-creed-revelations/Animus_D...
just to clarify, this aint my own work...
[EDIT. i know the ign guide isnt as good as you'd expect, but when i first looked the gamesradar one wasnt started. now that its done i hope you'll be able to use either to your advantage, apologies for the rough start but they had to find them without a guide first!]
Personally how i got this achievement was like this: As you play through the campaign, try and avoid collecting them (makes going through guides easier) but just scan them with Eagle Vision, that marks them on your map. then when you have 50 marked on your map/collected you'll get the rest added to your map for FREE! you'll know when it happens at it'll pop up in the bottom corner of the screen saying map updated. (thanks to kevgeotech for confirming this)
p.s. i would also highly reccomend C Frizzady Diz's guide as well, just had a look and looks really good.