What is the best way to get a merc with freeze aura, or is there a better way to use the mercenaries when playing with necromancer (main attack: bone/spirit spear)?
Act 2 Mercenaries auras are based on the difficulty and then the label they have (Combat, Defense, Offense), here is the list:
Combat (Normal): Prayer Combat (Nightmare): Thorns Combat (Hell): Prayer Defense (Normal): Defiance Defense (Nightmare): Holy Freeze Defense (Hell): Defiance Offense (Normal): Blessed Aim Offense (Nightmare): Might Offense (Hell): Blessed Aim
As you can see, you want to
Buy a Defense Merc from Nightmare to get Holy Freeze.
As for other options, as Artless said, Act 3 Cold Mercs are great with FCR items, since they don't need mana they can spam their Ice moves all day freezing enemies.
Personally, I prefer a well built Act 2 Defiance Merc (Normal or Hell). These guys are super tank and with a bit of life leech and a good damage weapon (Bonehew w/ shael and a life leech hat is good for starters) they almost never die but also provide you with some defense as well (through Defiance).
Another good option if you can get a good weapon is a Act 2 Might Merc (Nightmare offensive) These guys die a bit more (than the alternatives) but can output great damage along side your bone spells, although their aura doesn't benefit you much directly.
The reason I shy away from Holy Freeze is that Bone Necros have fast moving projectiles, so hitting enemies isn't tough, and they usually are packed with FCR and an Enigma, which allows teleporting around enemies to avoid getting hit pretty simple too). Keep this in mind when reviewing my suggestions.
It's been awhile since I've played D2; the mercs with Holy Freeze are the "Defense" labeled henchmen in Act II in Nightmare mode.
As for other options; the "Cold" damage Iron Wolves from Act III have Glacial Spike, which accomplishes something of the same thing, though they'll be stymied by "Cannot be Frozen" monsters where the aura mercs won't.