I believe you may be able to use a real steel trigger but i'm not sure. However, I've reccomended this store to others before and it's your best alternative, if something on your gun breaks. It's called "Combat Depot" located in Ohio (I believe)
www.combatdepot.com and they sell tons of gun parts, especially KJW, including USP parts (i've gotten a few parts from there for my USP.) Most parts are labeled with there corresponding part number in the manual, so make sure you have it handy when looking for the part you want. If they don't have the trigger, then drop them a line, i'm sure they can help you out.
Hope this helps
Bryan