How to obtain and load a good initial data set for object localization?
The ones that are available out of the box - 


Open Images Dataset (Object Detection Track)
MS COCO dataset
Indoor Objects Detection
ILSVRC dataset 


Create your own


Search for the object of interest on Google Images (like bikes, cars etc.)
Use the Fatkun Chrome browser plugin to download all images at once
Use LabelImg to annotate and generate a custom dataset
Use pre-trained models like SSD MobileNet to re-train your own models


Might not be an exhaustive one, but it will definitely get you started. Cheers!