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Any farm supply. You might try a Sears Hardware if you have any in your local.
Are you just looking for the keys for the crank? If so here is a link to the ones you want (you'll need two). You could probably find them at any Chevy dealership.
The keys I linked above are .188" according to my mic. I know because I ordered a set when I ordered my Goodwrench engine, thinking I would need them, but it comes assembled with them. Maybe it is supposed to be a slight interference fit, but .02" or .028" difference seems like too much.