I grab a straight grained chunk of well seasoned birch off my firewood pile. I split off a piece of the desired thickness by putting an axe head in a vice and use a dead blow hammer to drive the block of wood onto the axe head, and trim it to width with whatever saw I've got handy. I typically leave the piece about 6-8" long so I can hold one end while I taper the other with the welder's multi-tool... 5" Makita grinder with a sanding pad. I generally make the taper pretty long, then just trim it to use whatever portion of the taper is the appropriate width.
My home-made wedges work better than the storebought wedges that I can get locally which invariably have too steep of a taper, and are somewhat fiddly to modify because they are quite short. Does not require anything special to make a good one from scratch. Its important to use nice dry wood, so once installed it will soak up a good amount of linseed oil and swell. Wet or green wood will only shrink.