I am constantly on the lookout for children's books in Spanish. I find them in Costa Rica, on Amazon, on my online book-swapping site (I've had surprisingly good luck there), at library sales, at thrift stores and once, incredibly, at a yard sale. I often--though not always--use money from my own budget in order to avoid financial ruin. We have quite a library built up, and I love seeing the boys get all excited when I come home and announce I've brought them a libro nuevo.
I found the Latin Baby Book Club last year and was thrilled to finally find a place that had useful, thoughtful recommendations for children's books in Spanish. I'll be writing monthly reviews there, and my first one is up here. The book is called Poemas con sol y son, and it's one of our favorite books of poetry to read together.
Just FYI, in case anyone is interested!