You can still find them if you look and if not I expect they will be back in stock soon if you don't have an immediate need.
As @nathanu says, you can definitely make you own out of Tyvek or similar building wrap. If there is no construction going on near you, you can find conveniently sized pieces for sale online. Tyvek generally comes out weighing similar or a little heavier than factory. "Polycro" and also duck windows film which is similar are other materials that people use. It's lighter but a bit harder to work with and prone to sudden splitting if you get a nick on the edge. Both materials are very loud if they move around. Tyvek can be quietened in a washing machine but becomes less water resistant.
I have made a footprint from Tyvek and it works ok. I put Coghlan's Snap-N-Tap grommets reinforced with packing tape in the corners of mine so I can stake out the footprint. I had to use a hole punch. But some don't bother and just cut it to shape. It should be slightly smaller than the floor of the body of your tent so it doesn't catch rain water and pool it under your tent. Carry a separate piece for a door mat if you think you need one.
But honestly I prefer the factory footprints because they are easier to use, quieter and they are more pack able than Tyvek which in its original fairly stiff form can only really be folded. Factory footprints usually seem expensive for what they are but Nemo and Big Agnes footprints give you a fly only setup which adds a useful feature to your tent mitigating the price somewhat. Doing this without a footprint is possible but too annoying to bother with. With BA you can also do fly first/last but I'm not sure about the Nemo. Basically putting the body up or taking it down from inside the fly helping keep the body dry when it is raining.
One other thing with footprints is to make sure to uses them the same way up each time. Factory footprint generally have an intended top and bottom side. If you make your own choose a top side and mark it as such. This will help keep the dirt and grit away from your tent floor. One of the ways a footprint works is to allow the floor of the tent to slide against a smooth material so it does not not abrade. Minimizing the dirt helps with that.
I am of the opinion that two light weight layers for a floor is a better system than one heavier layer and a tent like a the Nemo Hornet is really designed with that in mind. You should definitely get some kind of footprint before taking it out.