This was recently discussed
here. As a result I ended up buying a box of 200 Max-Lite plugs from
Safetysupplies. It worked out at about £23.21 after tax and delivery. That's (furrowed brow) 12p a pair! I've tried them out on a couple of ride-outs and find they work better than the ones I used to use. (Plus I'm not tempted to risk infection by re-using plugs just because I've run out)