Rihanna’s sightseeing dress, the first dress, is this Lucretia Silk Maxi Dress by Diane von Furstenberg:

It retails for $695.00 at Net-A-Porter.com.

The shorter, bell-sleeved version, the Diane von Furstenberg Tabalah Dress, is available for $425.00 at Shopbop.com:

Diane von Furstenberg Tabalah Dress, $425.00

Rihanna’s long-sleeved tie-dye dress is a bit harder to find and identify, but I did find this maxi dress (sans the sleeves) with a very similar print:

Patterson J. Kincaid Santa Monica Long Dress, $158.00