i'm looking into building my own as well and PVC is what i've found to be the best option. it's lighter-weight and will not warp over time, and IMO the best possible material. yes it's more expensive, but you get what you pay for.
i'm in IL, but the best place i've found near me is Menards that have 4 x 8 ft sheets. you can buy PVC sheets online, but i'm not too familiar with that.
i hope i helped in some way!!!