Do you mean something like this? Top left of........
An internet search will throw up lots more.
I do see a slight problem. As a RIB has no top sides the ladder will tend to swing under the boat as you climb up. Setting up and probably lengthening the legs so the press on the hull would be good.
Alternativly a rope ladder would do, or just a couple of rope loops over the side or transom.
I have heard of people using the cavitation plate on the outboard as a step. Just make sure the kill cord is not attached.