I cannot find these caps on the SRD, but I know they exist, else a Rod of Undead Mastery and the secondary effect of a Dread Necromancer's Undead Mastery would be useless. Where can I find what the default HD control caps are?
These are different things.
Animated undead creatures like skeletons and zombies that are created through the use of the 4th-level Sor/Wiz spell animate dead [necro] (PH 198-9) et al. have the control cap (ahem) buried in the animate dead spell itself: "[Y]ou can control only 4 HD worth of undead creatures per caster level. If you exceed this number, all the newly created creatures fall under your control, and any excess undead from previous castings [of animate dead] become uncontrolled."
However, created undead creatures like the ghouls and ghasts brought forth by the 6th-level Sor/Wiz spell create undead [necro] (PH 215) and the shadows and wraiths brought forth by the 8th-level Sor/Wiz spell create greater undead [necro] (PH 215) are not automatically under the creator's control and must be managed in a different way, such as by using the 7th-level Sor/Wiz spell control undead [necro] (PH 214) or by successfully commanding them using rebuke undead (PH 159-60) or by simply forming an alliance, perhaps by employing the skills Diplomacy or Intimidate.
The rod of undead mastery (Libris Mortis 78) (10,000 gp; 5 lbs.) affects the first kind while the rod's held (and don't let go!). Whether the rod also affects undead creatures when they're controlled in ways like those needed to control the second kind of creatures is up to the DM.
Both kinds of creatures are affected in different ways by the dread necromancer's class feature undead mastery (LM 87), the dread necromancer both able to control more animated undead creatures and able to use the spell control undead to manage the more willful members of her undead horde.