I've bought bolts from Totally Stainless before. They offer a good selection of stainless bolts. I even bought a metric kit with metal bins that fit into a cabinet, and slide out. The only probem is they are not Japanese Industrial Standard. JIS which means they are like the european metric bolts sold at Home Depot and that came on Volvo's. The head sizes are different. 10mm is the same but a 12mm head on JIS is a 13 on the european standard.
My friend just bought some JIS assortments from Advance Auto Parts. They were labled metric bolt assortment for Japanese vehicles. They were Cad plated also.
Worth is another great source. You can get JIS bolts in the cadnium plating just like stock. They also have some cool shop supplies, like a spray lubricant that sprays liquid and get tacky after exposure to air, so it stays in place. And they have a good assortment of electrical fittings.
I've also bought bolts from Toyota, Thats what I prefer to do. You can get the correct bolt for the right item. It will have the correct strength and type of head. I think Toyota uses 3 strengths. The lightest duty one had a concave head and is used on sheat metal then there are ones with 4 and 8's stamped into the head.
FYI I did not note where you are from, but the salt treatments, or chemicals the put on the roads in the winter around here ruin the cadnium coating on the bolts. It kind of sucks paying for nice stuff just to watch it look like crap the next winter.
Some of the bolt sizes that I remember being comon are:
6mm x 1.00 thread pitch in 10,13,15,20,25 mm lenghts 10mm head
8mm x 1.25 thread pitch in 10,15,20,25,30 mm lenghts 12mm head
10mm x 1.25 in 15,20,25,30,35 lenghts 14m head
12mm x 1.25 and 1.5 pitch in 25,30,35 lenghts 17mm head these are not so common. I can only think of a few items that use them. The bolts that hold the trans to the bellhousing I think use the 1.25 pitch or fine thread, which screws you if you lose them, because most others are 1.5 pitch. Some bolts also are longer, like the thermostat bolts maybe and the ones that hold on the transfer shifter. That one is like a 8x1.25 in a 50, or 55mm length.
I remember that one is so long, because one year at a Catamount Cruiser Chalenge, I noted that the thermostat was leaking a little, so minutes before my group was going to convoy home (a 10 hour drive) I decided to tighten it Then the head broke off and it really started to leak. So I was able to pull off the top of the thermostat housing, remove the remaining part of the bolt with vise grips but could not find any other bolts that length. No one in camo had one. Then a friend ponted out the t-case shifter gate was held on by a longer one. after stacking a few washers under the head and using some instant gasket stuff from the goodie bag I was on my way home.
I hope that this helps some, sorry for the long post. Josh.