This looks promising:
You will need to contact the authors, as it is apparently password protected (if possible, consider posting the datasets on mldata.org for others).
You might also want to contact Ars Technica and Bryce Newell as they have acquired a lot of license plate images from city governments.
I would not limit yourself to a single dataset. You might actually want to first train on a dataset as ImageNet or take a network that has been pretrained on ImageNet. You can then replace the last fully connected layer (the penultimate layer). You can then restrict the training to this layer. This is fairly standard practice. You can then train on license plates and strings of characters and numbers.
You might also want to ask on https://opendata.stackexchange.com/ (in beta at the time of this writing).