You can try your lhs and see if they have durbro hardware they have good quality hardware, The size is 3mm x 10mm. do you remove the bolts alot? the reason I ask is because they strech when you tighen them and then the next time you tighen them they do not strech because they have already been streched and have become brittle. Try new bolts and blue loctite and don't murder them when you tighten them down.