You can buy polyvalent antivenom from SAVP and stock your own antivenom if you wish, but as BB said just make sure they give you serum that still has some good shelf life left before it expires.
Just bear in mind that it is not there for YOU TO ADMINISTER in the case of a bite but for you to TAKE WITH TO THE NEAREST HOSPITAL and not have to rely on them to keep stock for you.
Another thing to bear in mind is that the amount of serum required to treat a bite varies from case to case and it is pointless to keep serum if you do not have enough, I would say that 10 vials should cover most cases. 10 vials will be rather costly so it may be an idea to pool the costs between your buddys and store the serum in 1 central place.