Where can I get a new serial to rj-11 adapter to connect to legacy equipment (dsu, fx, etc)? I would prefer something that worked with the correct pinouts already.

I have only have one, and was having issues recently and realized that I had no other one to test with. It probably wasn't the adapter, but I like to have a backup. Over the years all the spares I had seemed to have disappeared.

Is this what you need?

https://www.amazon.com/InstallerParts-Female-Modular-Adapter-Ivory/dp/B00SNMS4OG?th=1

Coming back to this, yes I have seen those before. However, how do you know the correct pinouts? That's why I wanted to just buy one ready made - if that's possible.

Look in the Installation Manual page 2-35 for the proper wiring in this LINK

Well, that seems pretty straightforward.I'll have to get one and try. I hardly ever use the DSU / FX connections, but it's good to have a backup when you are left scratching your head as to why you cannot connect with a terminal.