There are several different variations of "Da Dao", big knife weapons. Guan Dao has a distintive ridge and hook shape on the back side, and has a spike on the opposite end, 5 foot pole with about 2 foot blade. There is a variation of guan dao which has rings on the backside instead of a hook. Pudao or horse chopper has a longer blade and shorter handle, closer to 2.5 foot blade and 4 foot handle.
Many Chinese styles teach a guan dao/polearm form or two, and maybe a two person set with a guan dao and another weapon. Taijiquan is not a style which focuses much on weapons, besides jian. seeing taiji spear or staff is fairly rare, I don't think I've ever heard of taijiquan style guan dao.
These weapons would not have seen much use outside of warfare, like the horse choppers name implies, it's designed to chop horse's legs. Guan dao was a weapon that would also be used mounted, as you probably know from the legend of General Guan and his green dragon crescent blade.
Like most traditional weapons, polearms are relics from a time long gone. It's possible to translate some of the skills involved to other objects or unarmed fighting, but mostly they are practiced for the sake of tradition.
There are several places you can order these weapons, just check any website which sells supplies for Chinese martial arts. I don't know if you can find a combat worthy weapon, but you can find them in "combat steel", which is heavy and more sturdy than the wushu type weapons. The purpose that this weapon occupies in most of the styles that still use it is one of conditioning, it's a big heavy weapon, and going through a long form swinging it around can be quite a workout for strength and stamina.