Look on eBay there are loads of places selling it. It's stria foam. Pass any building site in the uk and grab a bit. As for Vacuum bagging Fw recommend using a sand bag as an alternative. Look in the thread I posted a link to a PDF with good info on repairs. The key is let the board dry fully.
The deck skin is H80 corecell or devinycell I believe. The core is EPS/polystyrene. I agree with pmorgan, if you aren't already set up for vac bagging then the sandbag route is probably a better route.