Tours for Hobbiton depart from the Hobbiton Movie Set Store in Rotorua, the iSite in Matamata, or at the Shire's Rest just outside of Matamata. The latter is the closest to Hobbiton. The tour is exclusively owned and operated by Hobbiton, so you may as well buy tickets directly from their website. Tickets are also available for purchase on the spot at each pickup location.
We got ours day-of at the Matamata iSite, barely catching the last tour of the day (which was actually a great coincidence, apparently it was much more crowded that morning).
Tickets are the same price whether you leave from the Shire's Rest or Matamata iSite, so it may be worth it to your to plan to leave from Matamata unless you want to go out of the way a bit to drive to the Shire's Rest. There is limited parking near the iSite, although street parking is also available, and plenty at the Shire's Rest.