Your trimmer may or may not support them, so you may need a new trimmer. I picked my last one on the basis that it would support them...
Stihl (I'm just a customer) calls theirs:
- A "polycut head" (plastic blades)
- Two different "metal blade"
- Brush Knife
- Circular saw blade, scratcher tooth
- Circular saw blade, chisel tooth
The first three fit in in the "grass/weeds" job classification - the others get into brush/trees.
Walmart (and perhaps others) sell a thing called a "grass gator" which they claim "fits most straight shaft trimmers."
Realize that if one of these things contacts you, it's going to be rather more serious than getting whacked with string, and if one sheds a blade, it may become a hazardous projectile. The steel variety also add the possibility of sparks from hitting rocks potentially causing a grass fire.
As for search terms, I found the walmart thing (and many others) with "string trimmer metal blade" - and "string trimmer plastic blade" also gets results.