The B12 in supplements is made from bacteria and sourced from bacteria cultures. It is not from animal products.
However, some supplements can contain gelatin in the capsule, which you should look out for.
Streptomyces griseus, a bacterium once thought to be a yeast, was the commercial source of vitamin B12 for many years (8, 9). The bacteria Propionibacterium shermanii and Pseudomonas denitrificans have now replaced S. griseus (10). At least one company, Rhone Poulenc Biochimie of France, is using a genetically engineered microorganism to produce B12 (11).
Since bacteria are not animals, this should be vegan.
Also, if you don't consider this vegan, consider that eating bread made with yeast is more or less equivalent.