there is a roland part number, i know i bought some of the ones for my sc545-exEssentra Nylon Snap Rivet, 4mm | RS
uk.rs-online.com
not sure what size the 300V uses off the top of my head, but these are the sort you need in the appropriate size
They should be fine, there isn't really much can go wrong with them!I'm rolling the Amazon chinese crap dice, and I'll let you know if they work...