My gaming group has always taken a rather relaxed attitude toward the acquisition of new spells by magic-users, partly because we never subscribed to the idea of training between levels. As soon as a magic-user gained a level, it was PRESTO! New spells! I presume the rules for training were designed as an excuse for lightening the pockets of adventurers (which seemed to be a major preoccupation of game design in early editions of D&D). Personally, if I'm worried that player characters are becoming too wealthy to be concerned about adventuring, I have more plausible and interesting methods to part them from their treasure. I must admit, however, that if any class had a good rationale for training between levels, it's the magic-user. Unless their only means of acquiring spells is by finding them during an adventure (a severe restriction reminiscent of and appropriate to Call of Cthulhu), it seems there ought to be some explanation for how they suddenly gained access to x new spells.
Wizards' guilds are perfect for this purpose. A mysterious cabal of sorcerers united to protect their secrets and their fellow members, as well as to advance their esoteric interests (generally the gathering and safeguarding of occult knowledge), is the proper instrument for the guidance and instruction of magic-users. In any fantasy world where there are sufficient numbers of magic-users, this is the obvious place for magic-users to receive their training. Any town or city should have a wizards' guild, either secretly or publicly. Villages or hamlets, however, are unlikely to support such a guild.
The problem with magic-user training arises when the fantasy world has very few magic-users. So few, in fact, that there are entire communities, perhaps even entire nations that doubt they even exist! How does one get enough magic-users together to form even a single guild in such a world?
One could assert that all one needs is one's mentor to provide new spells, but there lies a host of other problems. What if the student surpasses the master? What if the mentor is faraway? What if the mentor dies?
In my fantasy campaign (IMFC), I have decided to grant all magic-users a special ability. This ability is not a spell and needn't be memorized. It is only used between adventures when the magic-user is in a place safe from immediate danger.
All magic-users, beginning at the first level of experience, are instructed by their mentors in the discipline of reaching the astral sanctum. This is an ability that enables the practitioner, through a process of meditation that places one's body in a state of suspended animation, to travel in one's astral body to a pocket dimension that is accessible only to others who have received the same instruction. Each astral sanctum is the exclusive domain of a particular guild, and it is here that magic-users are inducted into the higher levels of their order and receive their new spells. The astral sanctum enables the far-flung members of guilds to share their knowledge and even convene councils though their physical bodies are many leagues apart from one another.
The astral sanctum does not confer any other extra-planar abilities and does not grant the ability to astrally travel anywhere except the astral sanctum.
The length of time required to gain the new spells due one's level whilst in the astral sanctum is one week.