Currently stimulus templates are only accessible via NWB files.
For reference, you can access them as follows (example: allensdk 0.16.0, natural_movie_one
stimulus template):
from allensdk.core.brain_observatory_cache import BrainObservatoryCache
boc = BrainObservatoryCache(manifest_file='/path/to/manifest.json')
exps = boc.get_ophys_experiments(stimuli=['natural_movie_one'])
ds = boc.get_ophys_experiment_data(exps[0]['id'])
template = ds.get_stimulus_template('natural_movie_one')