I'm not sure how many medieval guildsmen were working on salary, but it would be few.
Actually, it was probably region-specific. In some places, guildsmen were free masters, and as such, their own employers. This is the model you assume. In other cases, however, most guildsmen would be bonded craftsmen and, as such, not free to provide service to just anyone - providing service would require the approval of their lord.
You can check HarnWorld, for instance, for a good example of that kind of dual system.
And I would expect that, the rarer the talent or field of expertise, the more likely the master would be bonded, unless they are powerful enough to just say to the lord "no thanks, I'll be my own master". Being bonded is a pretty comfy deal, because you are guaranteed lodging, board, tools, workplace, and a decent revenue. Which, all things considered, is more than most non-bonded craftsmen could expect (the last part, especially, was highly variable and subject to many vagaries). In addition, free craftsmen would require a steady stream of work orders, which is typically not what you could expect of spellcasting services.
I would think that most spellcasters would be bonded in some way, either to the local lord, a temple, or some kind of institution (the local city guard, the magical college...). Free casters could exist only 1) if they provide services that are in regular demand so as to ensure a steady income or 2) if they do not depend on spellcasting for their lifestyle expenses. I would not expect either to be the more frequent case, and free casters would likely only be available in major cities where demand is large enough - once again, you cannot rely on powerful patrons, since they would likely have their own bonded spellcasters for regular services, or some kind of arrangement with an existing structure.
This is not meant to say that free casters do not exist outside large cities. They do, but they do not provide spellcasting services for a living. Most of them either are adventurers themselves, or have a personal agenda and the means to support themselves without peddling their services. For those, we are back to the "you scratch my back, I scratch yours" kind of deal.
Your best bet is the people who *are* spellcasters but have a mundane occupation (the archetypal sage, scribe, or similar literate urban profession). The thing is, those people are usually not known for their spellcasting abilities, and might even have a good reason to keep them under wraps. You can access pretty much any kind of spell that way, but how does the provider know that you will not betray them when it is convenient to do so ?