Every new BC that I've seen includes the "power inflator hose" that you connect to your first stage of your regulator...
now... the above is for NEW BC's.
for most used BC's, the inflator hose usually isn't offered as part of the deal, but sometimes it is.... the reason that it's not typically offered is that hose is connected to the regulator set and often sold with it instead. Either that or the sellers don't bother to go get it and include it as a set.... I would say at that 80% plus used BC's aren't sold with an inflator hose, but that's just a wild add guess on my part. it might actually be higher.
Note, if you have to go buy a hose for your power inflator, make sure you get one for your brand/type of inflator connection. THere are about different types in the US of power inflator connections. mostly easily seperated by brand name.
for a good chart of what type uses what, go check out the Dive Alert horn pages. they make horns for all these connections and they have a chart with the "type" and the manufacturer name so you'll know which horn connection to buy. this will help you identify what hose you need for your used BC.