I'm pretty sure you can't get it unless you were to steal it from a lab. I read an article awhile ago about them turning something into a virus and injecting it so it would self replecate. Anyway I did a quick search and this is all I could come up with.
"The far future, and it might not be that far, is things that actually involve the genetics of muscle. One of the ways the cells work is that the DNA produces something called mRNA, or messenger RNA. Messenger RNA gives the instructions to produce certain kinds of proteins, and there are drugs called antisense oligonucleotides that wind up binding these mRNAs and stopping their actions. One thing that would be possible — you've heard of the myostatin gene?"