You're looking for the A/C line retainers right? There have been a couple different styles I've seen. The larger lines are the low pressure side if that helps. If you're near a Pick-n-Pull, there are probably dozens of them...
I'm not sure either of these are the right size but they come up in a search for "E350 A/C line retainer clip"
. I was going to run by the dealer but they dont have any in stock.
The clips aren't typically lost or replaced so its unlikely a dealer will have them in stock. Go to the dealer with the VIN, ask for an image of the A/C lines you're working with and show them what you need.
That'd be my advice but I tend to buy as much OEM stuff as I can.