Mountain Man- Since you are in california, look to see if there's an OSH within striking distance- they stock Grade 8 as open stock. it's the gold-colored stuff. Often it's fine thread, too. . . . which may or may not be a problem for you.
generally speaking, the larger the number of lines on the head of the bolt, the higher the grade. (for the 'ordinary' sort of SAE thread zinc plated steel bolt; the sort of bolt we're concerned with here will have its lines in a star pattern. metric hardware will often have an actual numeral stamped on the head. when you start talking bolts from specific alloys, head markings (when present) get arcane in a hurry.)
common examples:
no lines: grade 2. really soft; may be acceptable for fishing weights or stationary sheetmetal work. Sometimes used as 'shear bolts' for farm machinery. Easy to overtorque. this is the typical grade of all-thread, wood screws, lag screws and carriage bolts. unmarked metric fall in this catagory.
two lines, usually forming a right angle: Stainless, usually 18-8, 303 or other free machining alloy, and useful for many of the same things grade 2 steel is used for. (though the bolts are probably stronger than that, I wouldn't trust a hardware store stainless bolt for anything with a cyclical load. I'd rather order something specific and have it come from a traceable source.) 303 is too tough to use as a shear bolt, but still easy to overtorque.
three lines: grade 5. general purpose, used for bolting vices to benches, motors to mounts, etc. Metric 8.8 is roughly equivilent. sometimes you will find lag screws of this grade; good quality machine screws may be in this grade range.
six lines: Grade 8. often has a golden zinc-chromate or cadmium plating instead of the silvery-blue zinc of other grades. Used for bolting down pickup beds, mounting engines and accessories, trailer hardware, etc. Metric 10.9 is roughly equivilent.
there are higher grades, but they are usually specified for specific purposes and might be brittle or otherwise unsuitable for RE purposes. an example is metric 12.9, which may be used for low-stress engine internals (holding the oil pump in place, etc.)
be sure and get the grade of nut that goes with the bolt; hardware stores typically stock grade 5 bolts and nuts; those that stock grade 8 will typically have nuts to go with.