I use grade 8 on ALL suspension and steering components. Grade 5 is fine for things that are not under stress, things that your life is not riding on, or some things that have multiple bolts. Examples would be things like intake or exhaust manifold bolts, water pump/ fuel pump bolts (you'll strip the threads out of the heads or block before the bolt breaks) or body mounting bolts (a grade 5 bolt is stronger than the sheet metal or fiberglass it's holding), etc. When in doubt, go grade 8.

I buy most of my bolts at the local ACE Hardware. They have a huge selection and prices are reasonable when compared to Lowe's or Home Depot.