Roll Control tank holders are best in my opinion, you can install them yourself. They fold down out of the way when you aren't diving. You can get them at a local dive shop.
Dive ladder, if you want custom for your boat, will likely be custom, and I'd check with Blue Coral, or any of the other well recommended fabricators here on the forum. Somebody might have a used one that will fit you, but not likely unless it's the same kind of boat.
you could also build a tank holder all you need is some PVC pipe and ropes. My dad and I made one and it works exceptionally well. You just put an old car mat or carpet piece on the deck to keep from scratching the gel coat. It cost next to nothing and can hold as many tanks as you want just increase the number of PVC sections, In its current configuration it holds 6 tanks without being tied or weighted down in calm water. In rough water I would recommend taking the top tank off the pyramid for less chance of rolling.