I just read on someone's blog that you can phone Compassion's toll-free number and ask to be put on their correspondence sponsor waiting list. I wouldn't think you'd have to wait too long, since I understand that there are many children who don't receive letters.
To whomever may be considering being a correspondence sponsor I would say "go for it!" My correspondence children have been wonderful, I love writing to them and receiving their letters. Many of the children I have been assigned have never received a letter in their whole time of sponsorship and now are so thrilled to receive letters.
Even if you are just able to correspond with the child every other month, your letters will be cherished and your child will be so grateful.