BB:
I'm not sure how long is long to you.
Blank verse, which is unrhymed iambic pentameter (or could also be other unrhymed meters, I suppose), is frequently used for long English language poems. The need to maintain rhyme in English, which is a rhyme-poor language, makes rhyming longer poems difficult. Of course, blank verse has its own difficulties. The absence of rhyme increases the importance of other poetic devices. Most blank verse winds up sounding an awful lot like prose. I wrote a long-ish poem (490 lines) in rhyming ballad meter. Ballad meter is alternating lines of 4 and 3 stresses (think "Rime of the Ancient Mariner" or the poems of Robert Service), though lots of variations are possible, including the degree to which you rhyme.
Robert Frost wrote a number of poems in blank verse (iambic pentameter in his case), in the 200-400 line range, maybe longer. He is, IMHO, probably the best of the relatively modern poets in this technique. Such poems as "The Death of the Hired Hand" and "The Witch of Coos" are examples. These, however, may not be as long as you are thinking of.
Another thing I've done in a long book is to not write a single poem, but to break the story up in several poems, varying lengths, forms, style, voice, etc. This gives the poet a lot of flexibility, and I think keeps the reader's interest more. My book has 38 poems in a variety of forms: haiku, cinquains, short rhymes, rictameters, sonnets, the aforementioned ballad, villanelles, blank verse of 4- and 5-beat lines, and a 280 line poem in mixed meter (blank, sonnet, terza rima, and free-form). The whole book is about an hour read. If you'd like to have a look at how I handled this and if it would work for you, PM me. I'd probably want a simple assessment of the marketability and quality from a non-poet in return.
Otherwise, I'm sure William's suggestion could give you a number of good examples. The problem with recent stuff is copyright. Much of it is not available.
Best Regards,
NDG
