Plenty of generic stainless bolts about, but I would definitely recommend you stick with plastic/acrylic bolts. In one of my previous 'offs' I got thrown forward and smashed the screen off with my ribs, the plastic bolt heads just snapped clean off. This was good, as it meant the screen survived n one piece with only tiny scratch and I only had a minor fractured rib. I hazard that stainless bolts would have put up more resistance, resulting in more plastic damage, maybe more rib damage...
The only downside with cheapo generic acyrilic bolts off ebay is the heads can discolour from the sun after a year or two. But then at couple of quid for a dozen, who cares, just buy some fresh ones! That said most sets I bought were oten a bit on the long side, could never be arsed cutting them down, so they poked through by about 1 cm on the inside, minor annoyance.