Here is how I knocked this out just fine from 95% collection or so which I had from getting the other achievements. I did this after completing the DLC, and like the first poster said, that makes things more complicated, but you can still do it if you follow these steps: Open the links below in two tabs in your browser or two windows, whatever you want, as you're going to need to use both of them to figure out if something is in the DLC and then, if not and you need it, where to find it will be in the second database.
Use ezekiel 08's link to this database with the size and name of every object:http://s9.zetaboards.com/Katamari_Damacy/topic/331190/1/
Most importantly, ---DLC OBJECTS WILL BE IN RED TEXT--- in the tables, so you can go through your collection catagories that aren't complete and figure out if the objects you're missing are from the DLC and, if so, you can skip those objects because you do not need to get them for the 100% achievement.
Go through the collection catagories one at time in your collection list (use the big gumball machine looking thing on the main island) and when you find a greyed out item meaning you do not have it, use the database above and if it is a non-DLC item you need, then use the database from Loki Artemis' post to get info on the location of the grand majority of items:http://www.mcfallweb.com/xbox/beautifulkatamari/?ITEM=JDGDHD...
Make note on the first database that certain items marked with an asterix * have SUB-catagories like the pommel horse which has ten internal pieces or the toolbox which contains more tools which you must collect, but dont appear on the main screens in their catagory. If you go to the entry for these "container" items it will tell you how many pieces out of the total components you have. If you interact with them it will open a sub-menu that shows all the internal collection pieces like the regular windows in the collection screen. The database tables are in order of the collection list in-game, and if there is an item in the table with sub-components, that will be right below the catagory table for its main catagory with the object's list of sub components. To find these sub-components, again, use the second link and click on the main object (such as the pommel horse) and people will have detailed where to find each internal piece, usually.
As a last resort, if the second link's info on where the item is is vague or there isnt any location listed at all, go and do a google search along the lines of:
"beautiful katamari" "young monk" location
If you were trying to find the young monk. There's a bunch of different forums where people cry out for help on specific items and other people may reply with locations, and google will find these posts. I didn't have to do this often, as the second database is pretty darn good.
It takes quite a bit of work, but using this method I tied up main game collection list from 95% to 100% in two full evenings of play.
Note: please comment or PM me before downvoting. I know my summation here is a refinement of the above solutions- and not to be an ass, but at least note that I'm the only person who actually HAS the achievement who bothered posting the exact method I employed to do so. This is not an easy achievement and I am only trying to tie everything pertinent together in one solution post that will get you this achievement in the most efficient manner possible because it is going to take between "quite a while" and "a stupidly long amount of time" to get this achievement and I'm trying to save people from the "stupidly long" situation time-wise which is only possible if you combine the information in both the databases.