4mm M4 bolts are pretty commonly used in stems, however, some manufacturers differ. Just go to any decent hardware store (bring one of the old bolts) and ask them to point you towards the stainless steel metric machine screw section (doesn't have to be stainless, but after you get stainless, no need to worry about rust). Some of the stems use custom bolts, but most of those customizations are merely cosmetic. As long as the bolt head is the same diameter, it should work perfectly in place of the originals. Sometimes the customizations are to narrow the bolt head to make it fit in a recess; if you can't find a bolt to fit in the hardware store, you can use a power drill and a file to turn down the head diameter of the hardware store bolt down to the right diameter.

Or you can go to your bike shop and pay $5 for a bolt you'd get at the hardware store for $1.25. They sometimes can get the replacement bolts.

Lastly, rarely is the rust on stem bolts more than cosmetic. Most of the time if you take them out and spray them down with WD40, most of the rust will go away and they'll look okay again. Then, once it's clean, you can coat the bolt in a thin layer of machine oil to keep it from rusting again.