S.L.
My 3-yr old insists on having her sippy cup in her bed. So far we found Playtex Sipster (or you can get the insulator for insulation) to be the most spill-proof. They have valves inside the cup so water won't leak. They are BPA-free now.
http://www.amazon.com/Playtex-Sipster-Cup-Pack-Colors/dp/...