Home Depot sells LEXAN or GE branded sheets of polycarbonate, but the sheet sizes that they are likely to sell at your local location can vary widely. The one near me sells then 2 ft x 3 ft in three thicknesses. Other locations may only sell 4ft by 8 ft sheets in one thickness.
Lowes also sells it but will (correctly) list the thickness in millimeters.
Other potential brands you will see in your local store include Makrolon, Tuffak, Hyzod, and a few others.
As others already pointed out there are much cheaper options, from selling offcuts to re-purposing common products.
If you're just trying to make some long strips then you should consider aluminum strips or u-channel. Lowes and Home Depot have those near the threaded rod, yard signs, and mailboxes and you'll just need to cut it to length and add holes.
The little critters of nature, they don't know that they're ugly. That's very funny, a fly marrying a bumble bee. I told you I'd shoot, but you didn't believe me. Why didn't you believe me?