First, welcome to the forum.
To answer your question, first I'd ask, are you looking only for a replacement ladder that you can install, or, are you asking where to take it to have a replacement ladder installed?
If you're only looking for the parts, try Trekwood.com. They are a parts house associated with a large RV dealership. Their online parts store specializes in Keystone RV parts.
You can also go to this thread on the forum:
http://www.keystoneforums.com/forums...ad.php?t=12986 In that thread, you'll find several salvage companies that specialize in RV parts. While this is a "hit and miss" type of search, if you locate a ladder at one of them, it may be much less expensive than buying a new ladder.
On the other hand, if you're looking to tow your RV to a repair center to have the ladder installed, any competent repair center should be able to obtain a suitable ladder and install it on your RV. Ask around your community for opinions on which repair center is considered reliable. Many are "booked solid" during the RV season and likely can't work on your RV until the fall, but they may be able to take a look, order parts and schedule the actual repairs at a time that's better for you and for them.
Again, depending on what you want to do yourself vs hire someone to do for you will determine which route you would want to take.
Keep us posted as things progress. Your input may help someone else who is considering the same type repair.
As an observation, Keystone never "made" your ladder. They purchased parts from a variety of manufacturers and "put them all together as an RV" so even though Keystone doesn't have the ladder on hand doesn't mean much. The identical ladder may be currently installed on several other brands being built by other traielr manufacturers...