Here's an idea: no spell, but magic-users who want a familiar can use magical research, as if they were researching a spell. The "spell level" equals the beast's hit dice. It takes a number of weeks equal to the creature's hit dice to find the familiar; 2,000 GP per hit die buys you a 20% chance.
Familiars are based on ordinary creatures, but when the magic-user concentrates, the M-U uses the familiar's eyes, ears, and other senses. If the familiar is killed while the M-U is "linked" that way, use a System Shock roll or the equivalent; failure means the M-U collapses and is unconscious.
Optionally, a character can start with a familiar in place of a spell book. The M-U communes with the familiar to refresh spells, instead of consulting a book. If you want this option after character creation, find a familiar as above, then research "use familiar as spell book" as a separate "spell".
If there are exotic familiars like imps, the M-U still can only "find" a natural creature, but the GM determines the chances that the familiar is really a polymorphed imp or other supernatural being. It might take a while for the M-U to discover the truth about a familiar's full powers.