Whenever a player casts an artifact, instant, or sorcery spell, you create a Mutagen token. (It's an artifact with "1, T, Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery.")
No shop on TCG.shopping has this in stock right now.
Run a card shop?
Heroes in a Half ShellBorderless · Surge Foil96 · mythic$296.41
Leonardo, the BalanceBorderless · Surge Foil83 · mythic$263.78
Michelangelo, the HeartBorderless · Surge Foil92 · mythic$249.05
Raphael, the MuscleBorderless · Surge Foil91 · mythic$221.14
Donatello, the BrainsBorderless · Surge Foil85 · mythic$149.87
Splinter, the MentorBorderless · Surge Foil89 · mythic$106.88
Shredder, Shadow MasterBorderless · Surge Foil88 · rare$89.47
April O'Neil, Live on the SceneBorderless · Surge Foil84 · mythic$77.23
Ninja PizzaBorderless · Surge Foil93 · rare$69.45
Casey Jones, Back Alley BruteBorderless · Surge Foil90 · rare$57.43
Krang, the All-PowerfulBorderless · Surge Foil86 · rare$42.17
Big Mother MouserBorderless · Surge Foil97 · rare$40.63