You might be able to find the poem at the Poets & Writers web site. John Barth used that line as a title for an article he wrote for that journal. In the article, he discusses the work of other writers, including Dylan Thomas. I've pasted a link to that article below.
I couldn't find this line in any Dylan Thomas poems, however I did find this reference from a book of Dylan Thomas radio broadcast transcripts, from Thomas reading from the story "The Crumbs of One Man's Year":
Once, indeed, I wrote a poem with a friend that began: "All trees are oaks, except for fir-trees". The poem had continued, "All birds are robins, except crows, or rooks."
I don't know if this poem was every actually written or if it was just a fragment.
The full and accurate quote, to my knowledge, is: "Once, indeed, with a friend I wrote a poem beginning, 'All trees are oaks, except fir-trees.'"
—Dylan Thomas, On the Air with Dylan Thomas: The Broadcasts