This question already has an answer here:
I'm looking for a book that I once read where wizards were divided into different factions.
One of those factions had a foot as a banner and they provide the source (e.g. the magic) that the other wizards need to do their magical works. If done properly they calculate how much a wizard needs.
Since they can only draw on magic (but not apply the energy themselves) they have to walk whereas other wizards can float/fly. Hence their order name.
Which book am I thinking is this?