Mine Thrower Rounds are the ammunition used by Mine Thrower type weapons in Resident Evil 3 and Resident Evil 4.
Resident Evil 3
The Mine Thrower's ammunition becomes available after discovering the weapon in the Saint Michael Clock Tower. The darts can be found in various areas after.
Resident Evil 4
Ammunition for the Mine Thrower comes with 5 Mines per box. This Ammo is a bit rare, as enemies and crates tend to drop darts at random. The player won't often gather a lot of ammo in one playthrough.