B12 shots are a shotgun approach. B12 should be taken daily. Like Vit 'C'
you cannot OD- its excreted in the urine, keep your levels way up.
Methyl cobalamin sub-lingual is the best absorbed. Don't bother with cyanocobalamin unless its impossible to get methyl.
I get mine from iherb
http://iherb.stores.yahoo.net/b12.html