Cow's or sheep's eyes
Fiesta has lamb's heads with eyes included in the deal. Once you've procured the head you may walk over to the produce section and buy some maguey leaves if you're feeling really traditional.
Of course then you'll need some post hole diggers, a nice supply of cured American hardwoods and a good sized cast iron cauldron as well as a backyard to complete the process.
In about 6-7 hours you can have the hole dug, the fire going and the head wrapped in the maguey and ready for cooking.
Do some research and you can get all the particulars straight but the combination of the steam and smoke plus the flavors of the maguey combine to make some very fine backyard barbacoa.
While La Monita inarguably serves the finest in town I doubt that they're going old school in their method.
A related question might be: Who's serving traditional barbacoa using the method above?