Magic-users of Kaelaross are pragmatic when it comes to using spells - they will generally rely on what they know works. The 105 spells of the Labyrinth Lord rules (or the 117 spells of the Rules Cyclopedia) are the standard spells of the magical colleges. These are considered the benchmark for reliable spells, and any magic-user can gain access to them when he has enough power and experience. Even after the Wars between the Empires and the Summoning, a magic-user growing in ability can reasonably expect to find and learn these standard spells.
Beyond these there are many hundreds of other spells but these are far rarer and finding them is certainly not easy as the spell-books in which they are written are often the personal property of jealous wizards. These rarer spells have been researched and created by powerful mages, written down in arcane journals and grimoires for the wizard's own use - hardly ever intentionally passed on to other spell-casters. There is also the possibility that such spells may not work as expected. Nonetheless, acquiring a spell-book with unique spells in it is the ambition of many a magic-user, and is worth risking life, limb and friends for. Even a non-wizard who recognises a spell-book for what it is can sell it for either a lot of gold or for a significant magic item. Since the Summoning, when many wizards were killed either by monsters, enemy armies or rival wizards, some of these spell-books have found their ways into treasure troves and the loot of powerful villains. And if such villains are capable of learning and casting such spells, retrieving the books could be very dangerous.
These "unique" spells can be taken from whatever source the DM wants, including other editions of D&D/AD&D, OGL products and game supplements. I particularly like Ed Greenwood's Pages from the Mages, where spellbooks often have themes that were of interest to their sorcerous authors. The idea that they can be introduced by the PCs finding a spellbook means that the DM has some control over what new spells the players have access to. The DM should always have final say as to whether a spell is used in his campaign, and can also raise the level of the spell by 1 or 2 if it seems slightly overpowered but not game-breaking. Compare the new spell to the standard spells from the Labyrinth Lord rulebook to see what level it would best fit.
If the character has the time and resources, the DM may allow the character to research their own unique spells. This is by no means a certain method, and failure can be frustrating and potentially dangerous. Similarly a character may try to use research to recreate a spell he has seen elsewhere. This may work, or it may result in nothing at all, or it may produce something either sub-standard or a spell that the character did not really intend - serendipity is an interesting source of new spells.