I knocked out what I consider to be a pretty decent sonnet the other day. I have no idea where one sells a sonnet, if one is so inclined, or what sort of people broker poetry out to those who need a fix on it.
I enjoy poetry, and I will consistently give the advice to anyone who asks me that if they want to improve their prose, they should write a bit of poetry. It's prose concentrate, without the necessity of an immediately coherent narrative, and freed from the shackles of strict grammatical prescription.
I mean all poetry too. Sonnets, limericks, and haiku to get a sense of rhythm and economy in a confined space. Free verse stream of consciousness to work on punchy, concrete images, and the rubik's cube magic of juxtaposed reference. Lewis Carrol or Seussian nonsense poems just to play in the sound of the language Dense, singularly focussed works around one image or feeling, with every word serving toward that sole purpose.
When I find myself unsure or disappointed in my own prose, I always go back to poetry. I've found it's served me well.
Poetry Hunter is a decent reference, by the way. Here are a few of my favorites:
Ozymandias by Percy Bysshe Shelly
Pequeño Vals Vienés by Frederico Garcia Lorca (Little Viennese Waltz in English, but if you can read Spanish even a little it's worth it for the sound and rhythm of the original. )
Invictus by William Ernest Henley
Jabberwocky by Lewis Carroll
And because a friend sent me a really delightful response poem, the original: To His Coy Mistress by Andrew Marvell and To His Importunate Mistress by Peter DeVries