You don't have many choices for the 335 and the BMW Perf SS is probably the best bet.
Rogue Engineering pulled their SSK
UUC has an SSK for the 335 but it's expensive and it sucks.
B&M has one that's affordable but I don't know much about it.
I went with the BMW Perf SS but have not installed it yet.
2011 Mineral White M3 ZCP
Stock for now