Adding to the list of Evergreen & Plastruct.
Tamiya also sell a range of styrene sheets in different thicknesses both in white or clear, as well a sheets for vac-forming, etc.
Personally, I find Evergreen and Plastruct pricey and will only use them for specific applications.
I use the Tamiya sheets to make my own stackable display cases for smaller Kits and Diorama.
Another option is going to a well stocked DIY shop and check around there they also often offer plastic or acrylic sheeting.
Depending on thickness and size you might also be able to use some plastic containers from your own shopping.