Out of curiosity, I pulled some bolts on 4 stems. On two of the stems, a Ritchey and a Profile Design, I was surprised to find out that the bolts used were 7mm x 1.0mm. This is an unusual size; try googling it to find a source. Most likely, made to order for the OEM like most of these stem bolts seem to be.

The other two stems used 6mm bolts. One of them had a reduced diameter head (and a correspondingly small recess), and the flat washers under both of them were a reduced diameter. Measurements below.

For comparison, a standard 6mm bolt has a head diameter of approx. 9.9mm and a 6mm flat washer has an outside diameter (OD) of about 11.7mm. A 5mm flat washer is about 9.7mm OD, and 5 and 6mm split washers are about 8.8 and 11.2mm respectively.

Bolt 1:
head dia.: 9.0mm
washer OD: 9.3mm
stem recess: 9.4mm

Bolt 2:
head dia.: 9.9mm
washer OD: 10.1mm
stem recess: 10.3mm

I think the conclusion is don't expect to find drop in replacement hardware for most stems. At least be prepared to get your calipers out and check.

If you're lucky, the stem uses 6mm bolts and has room for the head. Then maybe you only need to deal with the washer. Other than that, I guess its firing up the file and grinder or searching the LBSs and spare parts bins.