In any area with a large population of magic-users, there will generally be a wizards' school. This is a place where mages can learn new spells without having to track them down themselves. There is generally a fee of 100 gp per spell level for each spell that is learned. In addition, most schools will pay very nicely - typically at least 1000 gp per level - for any new, useful spell that they do not currently possess.