you may begin to contact some OEM thanks to alibaba.com : just search "android tablet", select the products that appear to be "acceptable" and contact the company! Many of them can easily work as white-label and personnalize their products if you buy a few hundred of them...
but pay attention to quality! (you won't work with samsung but with small companies that have the propension to build bad/cheap products). Try to work with an experienced guy who knows how this type of sourcing works!