I think this post may help; there are 3 places listed, one of which is the Lippert doors, as bonkers14 suggested.
Note: When asking for a door size, the door manufacturers normally go by the rough opening of the camper's wooden frame, not the physical door itself. The odds are high, on a new door, you have to buy the entire door frame to fit in the rough opening, the door itself, and the screen door if wanted as a complete unit.
Ebay, also has some, the selection is hit or miss but sometimes you can get lucky.
Hope this helps,
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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