Looks like you can purchase up to 25% of the miles you need at $32 per thousand miles (http://www.continental.com/web/en-US.../buymiles.aspx). So 25% of 50,000 miles is 12,500 miles at a cost of $400. That gets you to 48,000 miles. There are all kinds of "nickel and dime" type promotions to grab an extra 2,000 miles. That said, though, you could purchase a one way for around $860 (per AA's website), so the $400 buying miles option seems a bit expensive.

I don't think Continental trades on points.com, or I would suggest that too.