Getting yourself a singlespeed is an exercise in self-flagellation.
You can go and find one pre-existing. Good luck, they are out there.
You can also conjure one up yourself. Any mountainbike can be converted to singlespeed.
By far the simplest conversion is on a hardtail. Doesn’t matter what it is, nor how old, in fact an old crusty dunger is ideal in some respects. Trust me, brakes and suspension that are not state of the art will be the least of your worries when you are deep in the woods with no gear lever.
You can pull the derailleur off the bike, along with the cable and gear lever, and remove the cassette from the rear wheel. A singlespeed sprocket can be bought for bugger-all, and it comes with some spacers designed to slide on to your cassette freewheel where a gang of sprockets used to be. Procure and mount a straight cut chainring (ideally a 32 tooth). Shorten the chain, or replace with a cheap new one.
Now your only problem besides having to ride the thing is functional chain tension. That greasy old derailleur you binned didn’t just change gears for you, it also kept the chain at an optimal tension, and those spring-loaded jockey wheels took up a heap of slack.
The complicated but awesome solution to this aspect of your build involves tracking down a steel frame. Not common these days, but if you can find one you can take it to a reputable frame builder (an even more endangered species than a steel framed mountain bike) and have the dropouts switched out for something allowing some adjustment of the position of the rear wheel in the frame, to take up any slack in the chain. Or even crazier, have them install an eccentric bottom bracket. You can then rotate the thing to the right position and ride around looking smug, at least until you have to ride up a hill.
You can also buy, for not much coin, a device that mounts where your derailleur was, and is a spring loaded jockey wheel that makes your chain run correctly.
That old bike you never ride but is not worth selling can become your singlespeed, and you will have a thing you can use to beat yourself into shape, improve your strength and generally torture yourself every winter.