Where can I get unusual dowel/shelf bracket sizes?
I'm not really sure what you're describing, but have you considered the originals may be metric (9mm, maybe).

Furthermore, if you're thinking of using dowels, you should be able to either:

1)  Get the next size up and sand down by hand with rough sand pape until you're able to jam it in, or
2)  Drill a slightly larger hole in a size you can match, or
3)  Use the next size down and fill the gap with glue (epoxy would be good because it dries very hard)