my recommendation would be to purchase the land and old cabin separately. this gives you the most optionality and, most likely, the best end result.
there are a few places to purchase existing cabins, one of which is here: http://www.oldbarnwoodsite.com/old_log_cabins.htm
for land, without doing your own research, i would caution against starting by engaging small realty firms who will often push a limited subset of property that they directly represent. you can check MLS listings yourself for free by doing a google search for "MLS search virginia". a variety of websites will let you search all properties by just providing an email address. larger agencies like Remax.com will also have pretty wide coverage.
you'll also want to be careful to be familiar with zoning / code / etc. restrictions on the land you look at, or else you might end up with an empty piece of land and pile of old wood. the land sellers will not go out of their way to make you aware of these things. it's much more important to have a local real estate lawyer prior to making a purchase, than a local realty agency.