In order to unlock the HAMMERhead Prototype Outfits you have to unlock all other items in the pro shop.
This requires you to reach level 4 with all sponsors. This means EA Sports, Cleveland Golf, Taylormade, PING, Callaway, and Nike. Each level has various objectives you must complete to unlock the next level. You can view these from the Sponsor menu. Select a sponsor and press (A) to veiw the objectives for the next level. Note that you must have the sponsor "equipped" for the objectives to count. Select the sponsor you want to work on and press (X) to Set Sponsor.
You can also purchase level 4 of each sponsor by selecting them and pressing (LB) to open the store. To purchase all sponsors will cost you about $20 so I recommend playing though a bunch of seasons and unlocking as many as you can while working on other achievements and use this as a last resort.
*ATTN*=> Purchasing level 4 sponsorships prohibits unlocking the "Pin Collector" achievement, by preventing gaining the "Prototype" pin for legitimately unlocking the prototype clubs. Thanks BillJones0302 for the heads up.
If you have purchased Sponsor levels and it has prevented you from getting "Pin Collector" you can go to System Setings > Memory > Hard Drive > Games > Tiger Woods PGA TOUR 12. Now delete any items named "Tiger Woods PGA TOUR 12 Sponsors" from your hard drive. (You can redownload them again later if you wish.)
This ALSO requires that you do most of the Sponsor Challenges that are made available to you before each event. The more of these challenges you complete the less often they appear. So it may take you a few seasons to get enough of them done.
And last but not least you will have to win all the major tournaments to unlock the "Champion Outfits". This includes: The Masters, The Fed Ex Cup, The EA Sports Championship, The UK Open, US Open Championship.
The only thing you do not have to unlock is the "Level 70 all Stats" outfit, which is available by purchase only.
In order to see what you still need to unlock use the search option in the Pro Shop. Just set up the filters like this:
When you select an item it will tell you what you need to do to unlock it.