Legally what should I do to get my security deposit back?

So I found this rent by owner for a 1bd basement in the state of Virginia. Rent was supposed to be $1000 per month with $500 security deposit. A friend of mine was going to rent with me. In mid-April the landlord verbally told us that she basically chose us to rent to (it was a craigslist ad). I asked about signing a lease, but was told the lease wasn't ready yet. I stopped by a few days later and paid $300 of the security deposit. I have a signed paper from the landlord stating that I paid her $300. I was supposed to stop by the next week to sign the lease with my friend and pay the $1200 I still owed them. Well, two days before I was supposed to stop by, my friend told me she was leaving the state in order to take care of her uncle, since he had cancer. I was out of a roommate and could not afford the rent now. I called the landlord right away and told her what happened. I asked to stop by a few days later to pick up the $300 I gave them. She told me she was not going to give me the $300 back. They had paid their $2800 mortgage payment off that money and could not afford to do so. Since it was about 4 days before the first of the month, they weren't going to be able to find another tenant to rent to. So I was basically homeless, since I had to find another place to rent in a few days time since my other lease was up. I tried to bargain with them, asking for $200 of it back, just something. I kept getting told "no". How do I get my security deposit back?
Added (1). Called a cop, he went to their house to try to get money back. Legally there was nothing else he could do. He told me to take them to civil court. It costs $98 to take them to civil court. Since I only have a piece of paper saying I gave them the money with their signature, would this be hard evidence to get my $300+$98 filing money back? Do I have any other options? Thanks
Added (2). Also, I might add that the landlord also told me after I asked for the money back that the security deposit was non-refundable. She had no papers, nor did she EVER tell me verbally that this money was non-refundable.

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