M2 to M5 stainless I usually buy on amazon, search around for the cheapest ones. Bigger metric bolt supply houses and you might have to order them and wait. Standard Bigger up to 3/8 for grade 5 I go to coop or Peavy mart. They are sold by the lb, price check is required. If I need specific fasteners, smaller standard like 6-32ec, Fastenal and local bolt supplies. Any thing bigger then 3/8's I usually go to the bolt suppliers. Most have walk in sales. If possible see if you can piggy back off of a commercial account, just like KMS tools this can give you a big discount.
Nisku bolt used to have grab bags that they would sell at the front door. They were gold for hobbyist as they were packed with lots of partial and odd fasteners. Have not seen them for years so might have been a different era.
