I think you're mistaken here or I probably wasn't very clear in those other posts then. As the thing is, I also have had an SP-555 and for that I've always used 1Gb cards, hence the brand recommendation.
There should be small stores that still have 512MB cards stock, mostly the photo camera stores, but I've heard the 512mb line is discontinued, so you might have better luck trying eBay, Amazon and so on.
Perhaps NewEgg has some left.
http://www.newegg.com/Product/Product.a ... 6820191068
(Disclaimer: Not sure if this brand is fine, perhaps someone else can tell. )
As for the tutorials, I'd check out YouTube. There should be some really good SP-606 movies there. Also check out the Roland website;
http://rolandus.com/products/productdet ... jectId=573
(click on the 'video' tab there!)