Hey Mark,
Many tapers on the Forum have had good luck with the TalentCell lithium ion batteries sold on Amazon.
You can see all the offerings by searching Amazon for TalentCell.
Most (all?) have a 5V USB output that you'll need for your DR-40X. You'll need a USB A to USB Micro-B cable as well.
This 100 Wh TalentCell model is convenient as this is the maximum capacity that you can take on an airplane in carry-on without having to notify the airline ahead of time.
https://www.amazon.com/TalentCell-Rechargeable-11000mAh-20000mAh-Portable/dp/B01337QXMALarger capacity units (100-160 Wh) may or may not need pre-approval to fly, depending on the airline.
If the Wh (watt-hours) rating is not listed, you can multiply the output voltage by the mAhr/1000 to get watt-hours.