bar magnets could probably be found at grainger.com or mcmaster-carr supply houses. i got mine from a job site i was on years ago, so i don't know where it came from. you don't have to use a bar magnet...just cut a piece of 24 gauge sheet metal (heavier if you like) to the size you require, and use 2 smaller magnets (available at staples or office max) to hold it in place.
the mercury thermostat that came with the stoker is not as sensitive to the smallest changes in room temperature as a digital one. the honeywell digital thermostat i bought at home depot for $39 is sensitive to a 1 degree change in ambient air (honeywell model RTH230B). just follow the 2-wire heating schematic that came with it (one wire from the stove connected to 'W' & the other wire connected to'Rh' on the thermostat) and you're good to go. it's 7 day programmable (if you wish) and there's other brands available which do the same thing.
the barometric damper's opening was 1/2 inch open, not completely half open, and it's main purpose is to save you money on whatever fuel you're using by keeping the heat in your appliance. by providing a small percentage of room air to go up the chimney instead of the heat in your coal stove, your stove is more efficient on fuel & stays hotter. you're heating the inside of your house better with it, than without it.
all the best,