If you know the manufacturer of the unit you can contact them
for guidance in purchasing a replacement.
If it is an older installation or you do not know the brand you will
probably have to replace the entire unit. Matching a new door
to an older door will be close to impossible .
Alternatively, if you can find someone with old fashioned door fitting
skills in your area they can probably get you a pair of new fiberglass doors
and fit them to the existing jambs.
Best of luck with the project!