Hello all. I have a 2013 Boulevard S40 that needs a new gas tank. Mine is badly rusted. Any recommendations on an aftermarket gas tank or where’s a good place to buy a new/used one?

IDK of a tank that just goes on except for the OEM ones. Some of the guys here have modified tanks and got them on it. Mostly inspired by the desire for more gas and fewer stops.
So, your skills are important if you're looking for a different tank,
Otherwise, your shopping skills are what matters.
There are motorcycle junk yards. You might call some insurance companies and ask them who buys their totalled bikes.
Granted, a wreck usually bends the tank up some. But not always. Dents fix, rust does too,depending on how bad. You might clean it up and they make a liner to pour in. IDK what a new one is, but I'm sure it's plenty. You can look in the Marketplace, on the forum. It's down in the list of places you can jump to.
You have a color you want? If you're gonna hope,hope Big.

I have 1 good condition 2000 green and white tank.  
I didn't see any dents and I haven't looked inside cause it is taped over.

PM me if you're interested.

David, First off I would say to go to Versy, that was my recommendation, and check out his tank. He has parts galore and has made an investment and effort to make this whole interweb thing we use possible. Without his effort, and a few select others, we would not have this incredible forum we have.  

I actually ride another bike about equally to how much I ride both my Savage/S40's combined. Plus I own 2 more in build mode. That other forum is not nearly as helpful, complete or comprehensive as this one. I am a mere interloper here, so in reverence to Versey, who helps make it all work, check out what he has first, if it doesn't work for you we can talk. He has provided stuff needed to me. I have a lot of inventory now, you might think I want to sell stuff and I do, but I also want to give respect to those who do far more to make this work.

"Ride the Savage Boulevard"

Plenty of websites are producing aftermarket gas/fuel tanks and motorcycle accessories on different platforms. Reach out some authentic ones and order that.