Rust can deteriorate a car door until it starts to fall apart from the bottom up. Getting a brand-new door is costly, and it might not be worth buying one to use for an aging car. An alternative to purchasing a brand new door is to get an older one from an auto parts store that specializes in supplying used parts that are still in good shape. Speak with an employee to make sure you can get an exact replica (if you can't find the same color door, you can paint it).
Salvage yards are an affordable way to find used car parts on your own. Instead of buying a brand-new part that hasn't been worn in yet—or a used part refurbished as new with an unfair price—you could find your own parts at a sometimes-lower price point. You'll need to have technical ability and the knowledge to pinpoint the parts needed, but you'll also need to know your way around a salvage yard.