I have a rhyming PB, titled You May Just be a Dinosaur coming out in Spring 2015. It took me 1.5 years to find the perfect publisher. What I did, before I submitted to a publisher, was search their website for rhyming PB's. This helped me gauge who was interested in this material. From there I'd look up the book on Amazon (from the look inside preview) or check out the book(s) in the library and see what types of rhymes in general they published. This helped a lot!
I agree it's difficult and the rule of thumb is it has to rhyme, well. The flow has to match throughout. For each sentence , I even matched syllables as best I could and always had an end rhyme.
Hope this helps and good luck!