I think your only solution for the PSR550 is to simply buy yourself a bunch of floppy disks while they still can be obtained and keep them just for your keyboard. While they can be hard to find I did find a seller on ebay.com.au based in Mt Waverly (because I know you are based around Melbourne Australia ) that is selling 120 disks with starting bid of AU$19.99. If I were you I would grab some of these while you can. The seller seems to have good feedback so looks reliable. And you probably have a good chance of winning this item because the seller won't post, local pickup only which limits the number of people that would bid on it. And he is only a short drive from where you live.
http://cgi.ebay.com.au/BULK-LOT-120-X-1-44-3-5-FLOPPY-DISKS_W0QQitemZ130285576022QQcmdZViewItemQQptZBlank_Media?hash=item130285576022&_trksid=p3286.c0.m14&_trkparms=66%3A2%7C65%3A1 %7C39%3A1%7C240%3A1318


[This message has been edited by Nigel (edited 02-08-2009).]