If they're typical, they'd be metric 5mm x 0.8mm-per-thread. A bike shop will have jillions of those. I'm not an expert in the idiosyncracies of older Bianchis, so I suppose it's possible they're something different. Did the guy at the bike shop actually try the normal 5 x 0.8 type and find that they didn't fit, or did he just say that without trying?