FedEx Office locations, ship centers, or even from supply cabinets at FedEx Drop Boxes.

: Sites like Amazon and Alibaba sell bulk packs of compatible heavy-duty cardboard mailers. Top Recommended "Solid" Envelope Options Shipping, Packing, and Mailing Supplies - FedEx

You can obtain "solid" or rigid paper envelopes for FedEx shipments through several channels, often for if you are using specific expedited services. Where to Get FedEx Envelopes