I found the locking fuel caps no problem. You can order them on eBay or from Raney's Truck Parts. The part number is FTA-C-13. The best price I found was $55.95 each.
The locking DEF cap was a challenge. It looks like Freight Defense is the only company that makes a locking DEF cap for Freightliners. Their part number is 100300. You can get the locking cap on eBay or Amazon or from the Manufacturer, but the best price ($43.95) is on eBay. It doesn't look like the non-locking cap, but I confirmed with the manufacturer that it does fit. I'll know for sure when it arrives later this week.
I think both of these are a must. My Freightliner dealer said he's had cases where people put diesel in their DEF tank and it cost $1000's to repair because it ruins the system. In fact if anything other than DEF fluid is put in it will do the same.