By and large, exosomes preparations vary from cell type to cell type. Therefore, serum source of exosomes may be appropriate for certain purposes. If the goal is to investigate human prostate-specific exosomes, the preparation that Bj shared may not be very helpful. Isolation of exosomes from human prostate cancer cell lines is relatively simple.
HansaBioMed is the only producer of exosome standards to my knowledge. They are available from different source, human biofluids or cell culture media from lots of cell lines. I suggest looking into their on-line shop: http://exotest.eu/online_orders/exosome-standards