Large wild shape is lvl 8.
The rules for milking are in DotU (RAW only for vermin and requires also Vermin Trainer feat):
Bestow Venom (DC 15): By succeeding on a DC 15 Handle Animal check to handle a vermin that has a poison special attack, you can compel the vermin to give up some of its venom. The creature deposits its poison into a container you indicate, providing a single dose. A Handle Animal check to extract venom takes 1 minute. You can then attempt a DC 15 Craft (poisonmaking) check to refine this venom into a poison you can use.As mentioned in the poison guide, check Elder Viper Tree from Expedition to the Demonweb Pits.
Some vermin forms are here:
http://coboard.wikia.com/wiki/Druid#Vermin_Wild_ShapeDon't forget this (from the poison guide):
Ways to possibly increase the DC of poisons from animals/vermin:
Bear's Endurance, Bite of the Werebear, or other con boosts
Inspire Greatness for more HD
Also, don't forget, that for poison DC scales with HD. When you wildshape, you're using your own HD for poison DC. I.e., creature's HD is unimportant (provided you can change into it). You're basically looking for creatures with interesting poison and high constitution.
Some forms from 2 random books I opened:
MM2
4 dire toad, Con 15, 1d6 Con
15 dire snake, Con 16, 1d6 Con
16 legendary snake, Con 24, 1d8 Con
Sandstorm
15 giant banded lizard, Con 23, 2d4 Str
9*/12 ironthorn, Con 25, paralysis
*with enhance wild shape spell (plant form)
Note that in wild shape, you don't gain feats (e.g. ability focus poison). Animal companions OTOH can take this feat.