15mm gears can be a PITA to find. at least with the 3mm keyway. (edit - 12mm gears are really a PITA, 15mm are just harder to find w/ keyway, but easier for sure.)
Finding HTD5 gears on ebay 15t is easy, here’s a 14t:
(no keyway - just use grub and loctite blue) Also aluminum so will wear out eventually.
12t (on 15mm still likely to skip unless you run an idler gear) - so not sure i recommend, but def drops speed down:
The best resource i know for these, but expensive, is beltingonline.com in the UK. Slow to ship and a not-inexpensive flat rate shipping… I think it was 25GBP for shipping last batch i got. But you can get good gears exactly as you want setup. Oddly not seeing 13t…
(adding 8mm bore, dual grub 3mm, 3mm keyway etc = ~$14 before shipping)
(again i’d only run 12t if idler gear to avoid skipping - even on 15mm)
14t (no 13t i could find 15mm wide):
(about $15 before shipping)
Good, if expensive, options…
HTH - GL!