The pink bikini can only be found in older versions of the game. Tomas had posted this link
in the past, this should be it:
http://www.mediafire.com/download/k2av6 ... rce.rgssad (Just backup the original LaxiusForce.rgssad file first, and then replace it with this one)
Bear in mind that is a very old release of the game, without any of the bug fixes and polish that have come since. There're a couple bikini items, and some very minor stat bonuses/items available by wearing it at specific locations, but you are left with a really old build of the game.
As far as the location, it's in Khome chapter #1, for sale in one of the shops on the left, but only on an old version of the game.