Chrome nuts, screws and soft metal
hehe, 2/8" would be 1/4"
anyway, the bolts aren't hard to find in a 3/8" length, you should be able to get them at any bolt supply house but as they are metric you're right, 3/8" is not available. *You need a 9.5mm long bolt. *You will probably need to buy a 12mm long bolt but a couple minutes with a Dremel tool and a 60 degree triangular file will shorten them up in no time. *I stripped out a nut once too, here is a cheaper fix, take the nut and pads to a machine shop and get them to drill out and re-tap the bolt holes to the next bigger size, I think I went with 1/4-28 UNF bolts. *These bolts are available in many lengths and are a little deeper thread that is harder to strip out.
No offense, but for what that luthier charged you, you could have gotten a new nut machined to the exact specifications of your old one and had it made out of a stronger alloy of steel that would also be tougher to strip. *Or for about $20 had it retapped. *I've done a couple nuts at our machine shop that the local music store sends over.