Since Doctors are not questioned as to why they might order such medications for their clinics (that's the professional latitude they're given... I've mentioned this previously), I seriously doubt Lance would be dumb enough to expose himself to such an extreme liability. But if the doctor gets caught doing dumb things with EPO, it ends 100% with the doctor. The patient retains anonymity according to the law. In the earlier days, these guys just got the drugs from the docs themselves. But the heat's on now. My guess is that the docs are getting the drugs, but they are sourcing the drugs from payoffs they give to the colleagues that work in Oncology, Nephrology (kidney medicine), etc. They're also smuggling it from other black market sources, and they then need "mules" to move it. It would be too easy to expose the fact that an Internal Medicine doctor is ordering mass quantities of EPO, HGH, etc. So the doc needs to source it quietly. They probably get the stuff from a combination of doctors and black market supply.
I'm guessing that Popovych is more of a "mule" than anything else. On the bike, he fetches water bottles. Off the bike, he fetches the "stuff". Perhaps his "Eastern European" (wink) connections make him especially desirable to Johan and Lance? Perhaps he's even more of a pipeline than a mule.
I'm willing to bet money that Popo is/was the main mule.