How can you buy XRP with BTC if you don't have any XRP? As far as I know the best way to buy XRP for BTC is from the Ripple client ... which requires some XRP to begin with.

If you own 0 XRP and some BTC, how can you get started acquiring XRP?
You can buy some Ripple (XRP) from Bitstamp. At the time of writing, they were selling 50 XRP / 1 USD.