Gaerlick has some options that are reasonably priced.
(this url hates me)
Here is something similar but I don't know what the mounting hardware looks like.
For installation, I'd always out some kind of backer instead of just washers, there can be significant loads on a boarding ladder.
I'd also recommend one that mounts permanently and can be deployed from the water, it really is a huge safety feature. A ladder that lives in a cockpit locker does you zero good when you are overboard. I could 'sort of' see having an short 'emergency' transom ladder and then a longer ladder that hooks on the side for other situations (boarding from the side in waves can be easier) but that seems silly compared to just having one that is always available.