I am playing a druid who has a viper as his animal companion. I'm finding that the DC for the viper's poison (DC 11) makes it less useful as we increase in level. Are there any spells or magical items that can be used to increase the DC of its venom?
The DC for Poison is a formula. The static value listed in a monster's stat block is a convenience so that GMs don't have to calculate it themselves.
From the d20 SRD:
So if you increase the viper's Con modifier (by spell, feat, item, etc.) the DC for the venom will increase. Off the top of my head Bear's Endurance or an Amulet of Health would both be helpful in increasing the venom's DC.
You can also have the viper take the Ability Focus feat out of the monster manual which ups the DC of a chosen special attack by 2.
Arsenic & Old Lace
For your animal companion, you can consider these options.
If you are willing to invest some feats and skill points, you can summon powerful vermin and milk them for their poisons, and then increase the resulting DC of the poison yourself. The absolute most powerful poison, however, is of a powerfully poisonous creature with the above feats and spells, milked, with the resulting poison prepared by a creature with the feats that apply to non-natural poisons, and then deployed in an area with the above spells cast.
Note: There is nothing stopping you adding a Collar of Virulent Venom and Poison Fangs to your Viper, thus letting it poison while it poisons and bite while it bites. Or as we in the business like to call it, bite primary attack routine (can full attack), bite secondary attack, both get to poison twice.
Spells that Increase a Venom's Saving Throw DC
Spells and an Alchemical Item that Penalize a Creature's Saving Throws versus Poison
Spells I'd Be Remiss Were I Not to Mention