You got the right idea. Where to buy will depend on if you are installing them yourself, or having a shop install them. Most shops want to supply them.
I bought my heavy duty Bilstein shocks from Tire Rack. CLICK HERE
for all the details which includes part numbers and pictures. Be mindful that our E350 is a 2007 model year. Verify that the same applies to your 2013.
You will want to upgrade to either Roadmaster or Helwig heavy duty front and rear stabilizer bars.
You will also want to upgrade your steering stabilizer to a heavy duty version that "centers" the steering wheel. I like our Safe-T-Plus with one exception. It did not come with a life-time warranty. It might today, but didn't back in 2007.
When the front suspension is placed on jack stands, I turn to steering wheel all the way to the left or right, and the Safe-T-Plus turns the wheel back to center. This helps keep the rig straight on the open road and also in the event of a front tire blow-out.
The Ford E350 and E450 stock steering stabilizer keeps the steering wheel where you put it, though very weakly so.