Virtually all EPO would find its way to athletes via prescriptions from friendly doctors (maybe veterinarians) or directly without prescription from friendly pharmacies (in countries where prescriptions are less well controlled). Drug companies themselves would not be involved, unless there is a "special" arrangement, which could have well been the case in the mid-late 80's onwards at Amgen with their cycling sponsorship. Amgen was quite small at the time from memory, and a senior management arrangement is not hard to imagine.
Most medium and large sized drug companies (eg Pfizer, GSK, etc) would not be interested in supplying illegally, it's just not interesting enough financially and the reputation risks being caught are just not worth it. On the other hand, there are small companies, notably in China, where regulations are still loose and "flexible", that will produce certain molecules "made to order". Of particular interest to athletes would be those drugs for which no test exists yet, ie analogues with metabolic pathways different to the existing drugs. Hence team managers (eg. Bruyneel) and doctors being highly interested in understanding the workings of WADA etc.
"Secret research" is possible in theory, but highly unlikely. For reasons stated above, but also as it's not that easy to access for athletes. One would have to have connections within the research departments to get access to drug supply, which is pretty well regulated and accounted for internally within the company. So someone inside the drug company would have to be in on the fraud and act on his/her own. To make the connection, ie for the athlete to know who to contact would be quite hard. Contrary to popular conspiracy thinking, by far most people working within the drug firms are of high moral and ethical standard. And the internal workings of drug firms, although not well known publicly, are highly structured and regulated. So a fraudulent rogue would not have an easy time, at least not in a medium-large drug firm.
I would estimate "off label" use for EPO (ie. use of the drug for indications for which it is not licensed, eg. athlete performance enhancement) could be quite significant, and may be estimated from epidemiology data on the diseases for which the drug is licensed (total number of patients with disease, and number of newly presenting patients per year), as well as sales data, which is available for some countries on the web via government data.
Contrary to steroids, I would estimate imported EPO not to be an issue in Australia. Remember EPO is a protein that needs to be kept refrigerated at all times. As such there would virtually be no illegally imported drug supply by airmail.