It's important to remember that "The Tyger" is a poem of imagination and symbolism, not a literal description of a real tiger. The tiger likely represents the powerful, creative, and sometimes destructive forces of the universe, and Blake uses it as a vehicle to explore questions about the nature of God and creation.
While Blake may have seen tigers in books, illustrations, or even zoological collections, the tiger in the poem is more a product of his own mind and a representation of his philosophical and spiritual ideas.