I usually get all that sort of stuff from s specialist fastener shop ( Boltmaster down here )
While a single bolt s always more expensive than our Lowes equivalent once you get into bigger numbers the price drops dramatically.
For things like washers I buy them by the box ( usually around 200 ) which generally works out around 10% of the single bolt price.
In fact I buy most bolts by the box load so I can charge them out at 50% of the mower shops price.
A LHT spindle bolt for a Husqvarna rear catcher ride on was $ 27.50 from the mower shop & $ 6 from the fastener shop.
Charged the customer $ 15 each for the one I fitted + spare and he was over the moon as he was quoted $ 30.25 by the mower shop + a special order fee + delivery.
You can also go with a high tensile flat washer ( thicker ) and a plain spring washer which will do the same thing.
I gather these are the ones that hold the spindle to the deck ?