Check first to see if there's a supplier of metal near you that carries the sizes used in the longeron in the same alloy, probably 6061-T6. If there is, then you can buy a replacement locally if needed. But often they won't carry the size.
The longerons are long and thin and light. It's easy to carry tied to a car or poking out a window, but easy to scratch, too. If you need to go get one, consider bringing along something to protect it with.
If for any reason you can't easily get one locally, then do consider ordering an extra.
Dave