You can purchase tickets to games three and four at braves.com (technically that’s at mlb.mlb.com, a subdomain that has sent more than one SEO dork off on a five-minute huff).
Orrrrr you could check Fansnap, which aggregates ticket vendors like Stubhub and the one million sites that are pretty much Stubhub. Game three tickets are available for as low as $18 as of right now, and game foursies are as low as $17 at the moment.