Tractor Supply is where I buy most of the bolts I need,most every time I go there I buy a pound of something like washers,nuts & bolts,even if I don't "need" them at the time...just to have them on hand..
--but their selection of metrics is quite limited and not heavily stocked,so when I need metric bolts I'm forced to go to the old "hometown" type hardware store that we have 2 of in a neighboring town ,and bend over at the cash register..they always have the bolts you need (and enough of them,instead of 1 or 2 like the box stores "drawers" to finish a project )...but they are over $1-$2 each!...ten bolts for my diesel pickup's exhaust manifold costed $20..but they were the only place that stocked that pitch & length of thread anywhere "local" so I didn't have much of a choice..
I hoarded a lot of "good used" bolts from tractors and things I've scrapped,and bought a few hundred pounds of mixed hardware at flea markets and auctions..quite a few guys sell them there to hopefully get more than they would for just scrapping them..and much of their hardware is brand new stuff..
I have litteraly a ton or more bolts & nuts ,but few are metric..most of it is grade 2 or 5,which I feel is sufficient for most uses on tractors and vehicles,I only use grade 5 or 8 on suspension parts or other critical structural applications..in some cases the "softer" grade 2 bolts actually work better on some things,they will bend rather than snap off under extreme stress loads..easier to drill one out if it does break also..
Several times I've bough 5 gallon pails full of mixed new & old hardware for $5-$10 ,I take them home,dump them out on the floor,and sort them all out..after awhile you have plenty of nails,screws,bolts & nuts,and lots of other stuff like toggle bolts,eye hooks,lag bolts,lead anchors,etc..one bucket I bought had a bonus,there was over 500 solderless connectors buried at the bottom,plus a new roll of solder that sells for $20 now by itself.
I also found over 50 of those blue concrete screws that are very expensive in the bucket too..
I now have more stock than some so called hardware stores do!...