If you are looking for a value on your property, and your lender is not involved, getting a CMA is better and cheaper. BPOs often do not allow agents to adjust for everything they need to adjust for, and you have no control over what agent does the BPO. With a CMA, you can interview and pick the agent, and there are no restrictions for adjustments. Many times an agent will do a CMA for free, so you save some money too! BPOs cost between $125.00 and $225.00 when you order them depending on how detailed you want them. There are several companies online that you can order a BPO through, just google "order a BPO" and you will get a bunch.
I have lots of friends in Greene County, right next to you. I would contact the TOP agents in your area for market condition graphs and reports. Call them directly - if you don't know who they are, call 3-4 of the largest brand names and have them give you the info. RE/MAX, C21 for VA. VA is where I started my career.
As far as a BPO...why would you need one? Get a FREE CMA or go for a full blown appraisal.
As far as market graphs, what locality/data are you looking for? You may find the link below useful - it is to market stats from over the years.
If you are just looking for an opinion of value for your home I would call a real estate agent. Most agents will do a CMA for free in hopes of making a connection with you and gaining your business. In, NC, real estate agents are allowed to do CMA's (Comparative Market Analysis) but not BPO's, as they are competing with the Appraisers. So you should just ask for a CMA.
If you are looking for a BPO because you may have a short sale situation on your hands, your bank will order it. They will only accept them from certain vendors on their form and they would not ask you for one.
If you look at Zillow, you can get a pretty good Market Condition Trend graph for your area or an address.