Target, wallmart and sometimes the larger drug stores usually have them. Honesly though my dd's were never comfy in a poncho until they were preschool age or older. If you can find a rain coat you may be better off. I just folded them (they fold up almost as small as a poncho) and put them in a ziplock.
I highly recommend a stroller cover if you don't have one already, those FL showers can pop up quick and soak everything in about 5 minutes
TJ