If you don't have a lot of (or care about spending as little of) in-game money as possible, get the FREE DLC packs called "Exclusive Unkut pack" and "The Ho Ho Ho Pack". With these, you can buy 93 items for only $1 each (in-game money) from a clothing store. After that, just buy some $45 bandanas and stuff to finish out the 100 items. I spent $408 this way. If you need quick cash, go to the Snatch mission where you steal hookers from rival pimps. Levels 1 & 2 of this mission nets you $450.
HEAD
-Santa hat (12 total)
-Unkut cap 1 (5 total)
-Unkut cap 2 (7 total)
UPPER BODY
Special T-Shirts
-Unkut T-Shirt 1 (8 total)
-Unkut T-Shirt 2 (6 total)
-Unkut T-Shirt 3 (5 total)
-Unkut T-Shirt 4 (6 total)
-Unkut Shirt 1 (1 total)
-Unkut Shirt 2 (8 total)
Over Shirts
-Unkut Polo Shirt (2 total)
Coats
-Santa Coat (12 total)
-Unkut Jacket 1 (1 total)
-Unkut Jacket 2 (1 total)
LOWER BODY
Pants
-Santa Pants (12 total)
-Unkut Trousers (1 total)
Shoes
-Santa Boots (6 total)
To get the DLC packs: from the main game menu screen, select Downloadable Content, then select Open Xbox Live Marketplace. When the DLC page loads, select & download the "Exclusive Unkut Pack" and then "The Ho Ho Ho Pack"