If you are currently attending a ostepathic medical school, they should have a library with at least some OMM videos.
Finally, see if there's a D.O. using OMM in your town. Nothing better than live and in person. If there isn't (which is very often the case), see if anybody knows a chiropractor who would be willing to let you shadow them. Let them know your plans to become an osteopath, and that you're interested in seeing manual medicine used. While there are some bold differences between OMM and chiropractic, there are a lot of techniques that are shared between the two, especially regarding HVLA, but this varies from practicioner to practicioner on both sides. When you come to school you'll see the real thing for hours on end in lab.