It's been surprisingly difficult to find a source for bolt spacers, the kind used to correct fender lines and space racks or fender stays with M5 bolts. Rivendell's fender installation video (at 3:15) shows an assortment, but my local shop pointed me to misc pieces and my hardware store only had 3cm spacers. I can only find washers and 5mm "bottle cage spacers" online. Anyone know a good source for these? Am I even using the correct nomenclature?

I really don't want to try to hacksaw longer spacers.

I just use a stack of #10 washers (same diameter as M5) to make custom spacers. You can buy a box of 100 of these at any hardware store quite cheaply. Get stainless steel washers if you can.

Try an Ace Hardware. If you have a Dremel you can always cut down Aluminum spacers.

Yeah, I figured washer's are commonly used. I'd really like to find single piece spacers, which I think look better and are easier to work with.

Yeah, I might have to try another hardware store. The one I checked was either Ace or True Value.

try McMaster-Carr, they have spacers available in many sizes and materials.
BTW they're not cheap

Automotive brake line and a tubing cutter.

Well, I ended up finding a place online that only sells aluminum spacers: Daven Screw Products. Better yet, they're made in the USA and shipping is cheap. Although sizes aren't metric, they provide metric conversions on each page. If these work out, I'll have all the spacers I should need for quite awhile.