Taxes, I think.

Real Knightly Orders usually administered their own fiefdoms, and also received some kind of stipendium from the Church. Inquisitors and the like got their salary from the Church. One would guess that WFB knights, templars and witch hunters get around with the same treatment - land properties' revenues plus Imperial stipendia for lay warriors, and priestly salaries for the templar and witch hunting fighters =)

Edit: Also, many knightly orders are made of wealthy noblemen and upper-class people, which are likely to contribute personally to the maintenance of their orders.