I need to put up shelves inside a custom shelving unit. However, the original toggles that held up the shelves were lost in a move. I was thinking dowels but I've been told that the originals were between 3/8" and 6/15". Is there a place where I can find inter-size, graduated or custom dowels? I've been to all the mainstream hardware stores and checked Amazon too. Anywhere else I should look?
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)
Almost certainly metric, assuming "6/15" is a typo and some reasonable inch size (5/16 or 7/16) was actually meant.
11/32 or 13/32 are lesser possibilities.
Wooden Dowels may be dubious depending on shelf loading - I gave up on finding 6mm shelf pins for an old "modern" German (Western) [yes, that old, at least, and that new, at most] shelf unit and bought 6mm drill rod (steel) from a machinist supply to replace the original plastic brackets (some were missing, other were expiring/cracking of old age.) I could find 5mm shelf pins/brackets easily, but not 6mm, at least when I was looking.
Depending what sizes they are actually between, 8, 8.5, 9, 10, 10.5 and 11 mm are all plausible for 5/16-7/16 - half of those go away if the sort out which fractional inch size you really mean rather than 6/15" (and 9.5mm is so close to 3/8 it probably would not be noticeable)