I went looking for little cutters like Peter was using in the Japanese Maple stream, and found some excellent ones.
A company called Valtcan sells a bunch they call “Model Kit Pointed Side Sprue Cutter” for around $15. The ones I got are hard carbon steel and work great!
“Valtcan Nayne One Pull Sharp Model Kit Pro Pointed Side Sprue Cutter (White)”
Search the biggest online superstore for “valtcan nippers” and you’ll find em, $21.
Update! The Valtcan nippers have a flaw - they can pinch your hand if you’re not careful–ouch! Check out these as well, no-pinch design, even less expensive, great quality:
Xuron - 2175ET Professional Sprue Cutter, amazon, $16