You should go to an insurance agent. NOW would be the perfect moment. Before you rent the warehouse. Because usually the amount of money you pay for your insurance depends not only on the value of your goods but also on the location of your warehouse. There might also be some requirements regarding doors, windows, security system and so on.
If you ask for terms and conditions now, you can take that into consideration before you sign a contract for a warehouse.
For example I pay a significant lower amount, because I am located inside the city, at a busy street, no windows to the back, only entrances to the front and the side window has grates.