TSC, Tractor Supply Company is a great place to find grade 8 bolts.
Couple things on your locking nut, see if a large flathead will fit into one of the slots, use a wrench on it or visegrips to try and turn it. Might brake a flathead but its worth a shot... I've also seen a regular lugnut spot welded onto one to use a regular socket to pull it. And, then again, I've also seen 'em blasted into bits with a cutting torch but it would leave a blackspot on the aluminum rim.
Or, save a lot of hassle. Stop at a local tire shop and say "hey, can you get this nut off for me?" chances are, they see that problem almost daily and have the tools to do it quick.