Take suggestions from the great poets themselves. For instance, you have probably read some of Langton Hughes's poems. (Remember that with metaphors one of the elements being compared may not even be expressed.) In "Dream Deferred" the title itself is a metaphor for aspirations that a person has.
In William Carlos Williams's poem "The Widow's Lament in Springtime," both parts of the metaphor are expressed in the first line: "Sorrow is my own yard." As the poem continues, some of the parts of the metaphors [unstated comparisons] are not stated, such as in thiese lines:
Sorrow is my own yard/ where the new grass/flames as it has flamed/often before but not/with the cold fire/that closes round me this year.
For one more example, here is a short poem by Seamus Heaney that contains metaphor also:
His hands were warma and small and knowledgeble./When I saw them again last night, they were two ferrets/Playing all by themselves in a monlit field.
As you can see, the comparisons made are unique and intriguing. Try to think of an unusual comparison that, at the same time, points to an odd similarity. See the enotes sites on literary terms, and be creative.