Take the existing one(s) out, wander off to your local bolt and nut supplier, hand it/them across the counter and declare "I would like to replace this, my good man!". Teeth sucking will no doubt ensue, but you should be able to negotiate the location of somewhere that can sell them if the first place is unwilling.
http://www.ba-bolts.co.uk/ might be a good start; they're in Cheltenham.