Pale Master cohort options

[Lasher Dragon]

OK, I've been playing a Pale Master since T&B, have since upgraded per the LM and I am in the dark on cohorts. In T&B it seemed pretty straightforward, the Undead Vassal had to be 2 less than my HD I think it was. This whole attracting cohorts is new to me, and I was wondering if someone out there had some experience with this and could break it down to laymen's terms. I mean, do I just kinda shop through the various monster manuals and find an undead type with the right amount of HD? Or is it a random roll thing? I'm 5 wiz/8 pale master right now, so next level is when I gain my cohort.

 

[Lasher Dragon]

Please if anyone can explain the cohort thing to me I would greatly appreciate it. From what I've gathered reading through the forums, a cohort is chosen by the DM, however I think the PC sets the CR or HD of the prospective cohort. So then does the DM make a list of possible undead that are of the correct CR/HD and then choose from there? Or does he just use the PC-set CR/HD limit as like the maximum, and I could get anything of that CR/HD or lower? That would suck...