Fortified Bicycle appears to gone out of business -- where to get one of their replacement bike light batteries?

First, hello, and welcome to Bicycles SE. Sorry to hear about your manufacturer collapsing... that's really unfortunate, particularly for a higher-cost item like an e-bike. My guess is that 18350950 mAh 3.7 Li-Ion is actually "18350 950 mAh 3.7 Li-Ion", meaning that the pack is comprised of 3.7 volt 950 mAh 18350-type lithium ion battery cells. This is valuable information, but also somewhat general. There are services that will open up battery packs like that and replace the dying cells with new ones. Batteries Plus in my area is an example.

If your light is not sealed, and you can see a battery that looks like this

(the colour/color does not matter) then you can pop out all the old batteries and fit new ones. Replace all the batteries in one light together, don't mix and match. The old ones should be recycled properly.

If the current batteries are soldered in, then its still possible to swap them, just takes more effort.

Aside, "18350" is a dimension, being 18mm across, 35mm long, and 0 means a round/cylindrical casing. 18mm diameter is slightly thicker than a common AA battery which is a 16mm diameter.

Matching the voltage and chemistry is important - you can't put a 1.5V battery in place of a 3.7V battery, because it probably won't work and it definitely won't charge safely. Matching the size helps stop things from rattling about and becoming loose.

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Additionally there is another size of battery called a 16340, which is 2mm smaller in diameter. This is also known as a CR123A, which might be an acceptable fit if you add some tape around the outside as packing. Note this battery can come as a disposable or as a rechargeable lithium-based chemistry. Don't recharge a disposable.