boosting my animal companions

[Teh_das]

I'm playing in a 3.5 game alongside a druid, a ranger, and an odd multiples scout/beguiler. I'm playing as a battle sorcerer, and I'm looking to build into druid and mystic theurge. I'll have an animal companion, and I plan to take improved familiar.

I want a blink dog with my improved familiar, and a phone kin for my companion, but I know they'll be weaker. Is there any feat or class that can help my companions?

[Kraken]

Have you considered arcane hierophant, from Races of the Wild? It's a theurge class that combines a druid's animal companion and caster's familiar into one being. So a wizard5/sorcerer5/arcane hierophant10 would have one companion that would be a 15th level druid animal companion with the abilities of a 15th level familiar added to it.

[eggynack]

First of all, use arcane hierophant. It's a class from races of the wild page 108, which theurges druids and arcane classes, and it combines features of your familiar and animal companion. You'll want a couple of mystic theurge levels, because you want to maximize caster level, and arcane hierophant has to be entered later, but you should maximize hierophant. As for feats, natural bond (CAdv, 111) will boost your effective druid level for the purposes of the animal companion by three, up to your actual level, which is usually comboed with high level companions, but might work as intended here, and companion spellbond (PHB II, 77) lets you share spells in an actually efficient way. Also, using exalted companion (BoED, 42) to give your animal companion vow of poverty (BoED, 48) is a classic maneuver.

[Biffoniacus_Furiou]

How many levels of Sorcerer do you already have? Ideally you would want to go Druid 3/ Sorcerer 1/ Mystic Theurge 4/ Arcane Heirophant 10/ Mystic Theurge 2, using Sanctum Spell or similar to qualify for Mystic Theurge early then retrain it to something more useful later. If you're not going to use early qualification tricks, go Druid 3/ Sorcerer 4/ Arcane Heirophant 10/ Mystic Theurge 3. Arcane Heirophant is miles ahead of Mystic Theurge, so take as much as you can of that as early as possible and only use Mystic Theurge to fill in the gaps.

Use this Sorcerer variant (http://www.d20srd.org/srd/variant/classes/variantCharacterClasses.htm#sorcererWizard) if you're getting two or more Sorcerer levels, you'll still count your full Arcane Heirophant plus Sorcerer level for your Familiar benefits per the wording of Arcane Heirophant. Get the feat Natural Bond in Complete Adventurer, which can also help make up for the lower effective level you get for a more powerful companion. Get a Fleshraker dinosaur in MM3 or a Dire Eagle in Races of Stone, both are 'level -3' companions and they're some of the strongest choices available. Make your character good/exalted, your companion familiar can also be good/exalted due to its increased Int score and it can take the feat Vow of Poverty to get some amazing benefits (your character should never take that feat, though).

[Teh_das]

I'm already sorcerer 5. I've taken the battle sorcerer and divine sorcerer (travel domain access). I have combat caster and battle caster as feats.

[Grams]

I want a blink dog with my improved familiar, and a phone kin for my companion, but I know they'll be weaker. Is there any feat or class that can help my companions?

You could take Exaulted Companion feat to get the blink dog.

[Teh_das]

Blink dogs aren't extraplanar, they're just magical beasts. I'll look into that though, because I know that feat gives more buffs than just imp. Familiar.

Wait, you mean as a druid companion? What book is exalted companion from?

[eggynack]

Blink dogs aren't extraplanar, they're just magical beasts. I'll look into that though, because I know that feat gives more buffs than just imp. Familiar
That's not really how the feat works. There's just a list of companions, and blink dog is on it, so you can have that. Also, VoP for all the random pseudo-animal friends. It'll be sweet.

Edit: Book of exalted deeds, page 42.