When Googling, it's helpful to know that the flower auction company is FloraHolland. Their website says that Connexxion bus 172 goes directly from Amsterdam to Aalsmeer. It appears to be a very frequent service: http://www.connexxion.nl/dienstregeling/lijn?id=M172&richting=2 There are four stops at FloraHolland, a very big place. I'm sure the driver can tell you which one is the Visitor Center - a dedicated entrance. We had a car, which was convenient. But traffic around Keukenhof is very, very heavy during the short Keukenhof season. If anything, busses might get some slight entry preference, so don't lament your need for public transportation. The grass parking lots are no pleasure on a damp day. I'd consider a tour company excursion from Amsterdam. However, looking at the Keukenhof site, you could consider going Aalsmeer to Schiphol on Connexion 858, and return to Amsterdam with the two busses. Do not skip Aalsmeer. It is fascinating. Their website mentions a local garden and a local boat ride, but we didn't do them.