Drops 1/86. I've done lots of RTs and have seen two PGF already.One with Larv Second with Largato today, the only thing you can do is keep RTing until you find it. PGF requires dedication.
Ok...I went through all 20 pages of PSOBB General to find these, so Chuk you can't say I didn't try xD [These are REALLY old ._.]
http://www.phantasystaronline.net/forum/index.php?/topic/2009-drop-rates-for-del-rappys/page__st__10
http://www.phantasystaronline.net/forum/index.php?/topic/2075-blue-odoshi-violet-nimaidou/
http://www.phantasystaronline.net/forum/index.php?/topic/872-luck-material-drops/
http://www.phantasystaronline.net/forum/index.php?/topic/1069-where-can-i-find-some-drop-charts/
You should ask Choko, he's really good with this kind of stuff as I am not xD