I have been giving myself B-12 shots for years and, I get my B-12 at the McGuff Compounding Pharmacy in California - you need a prescription for it but I have always had doctors who were primarily female and somewhat into alternative - therefore, I never had problems getting my refills.
Don't just buy B-12 from anyone on-line - you don't know what you are getting ...
Also, do not buy the Coblamin form of B-12 -
Methylcobalamin is a coenzyme form of Vitamin B12 which is biologically active. This means that your body can use it as is, and does not require any metabolic steps to make it body friendly. So, always get the Methyl form.