If the original bolts are a flange head then the replacement bolts and nuts should be the same. Washers could be used but the overall depth must be checked for clearance, this is particularly important on the shock bottom bolt. The bolt shank length and diameter also needs to be the same as standard.
Is it safe to use normal nuts and bolts from my local supplier for the rear suspension linkage on my Suzuki GSX-R1000? I can get all the nuts and bolts locally for the price of one OE bolt from the the Suzuki dealer.