A lot of the time, they just suffer without the aid they need.
I varies a lot though, depending on where and what your disability is.
Wheelchairs do exist in prisons. Wheelchair users often use the ground floor cells.
People with guide dogs generally don't need them in the restricted environment of a prison because they can simply memorize the layout like they would their home. Guide dogs are for more dynamic situations usually.
Serious mental disabilities often end up in mental institutions instead of prisons, they're often found not criminally responsible for their actions.