I just posted a note on another thread of my experience with Amazon selling Radwagon compatible batteries.  ebikeling does sell on Amazon.com and eBay but at a premium vs direct from them, so I just purchased the battery from them directly.  It included a new charger as well.

There might be a benefit of getting a battery without the vendor's logo on Amazon.com though, so it might be worth it to you. 

After waiting for the official battery to be stocked again, I gave up and looked for alternatives out there.

The main seller (with customer comments about RadWagon compatibility) on Amazon list the item in stock and then cancel the order because they actually don't have them in stock. Do not bother.

I think as long as you search for a "Dolphin" style battery pack, you'll be able to find compatible or higher capacity battery packs.  I found my battery on directly on the ebikeling site where they shipped from Chicago.  Customer service was atrocious (asked about batteries without their logo and they said they couldn't answer - this was a simple yes or no question) but the product itself was fully compatible with our existing base.

I bought the "48V 12.8Ah Dolphin LG Ebike Battery" if that helps someone who needs a new battery. 

Turns out the battery had the logo on it. Tempted to see if nail polish remover will remove the ebikeling logo but not in a hurry and may wait until the warranty is up before doing anything drastic.

We've been using the battery for a couple of weeks now and are happy with the upgrade.

Just a heads-up if you're in the same place.

Hmm - I've got an early RadWagon that doesn't seem to have any options for batteries.  I see that the Bolton controller upgrade is mentioned, which may open up the chance for an alternate battery as well.

Does anyone here have a battery rebuilder they recommend on the US West Coast?

Just bought a RadRunner Plus but need to keep transporting young passengers in the back too. :-)

Thanks!