On a recent chassis rebuild, I bought everything needed from the local Ace hardware store. Used all Grade 8 hex bolts, washers, and nuts.
Granted, it took a few trips to finally remember to get everything but it's not too far away.
On key suspension mounting points, I wouldn't recommend stainless bolts which are more hard (brittle) and could fail especially if overtightened.
BTW, Grade 8 bolts are coated to resist corrosion. Most all original GM bolts were Grade 5 or lower.