Growlithe can be found on Route 35--that's between Goldenrod and Violet. You shouldn't have any problem finding them since they don't exactly blend in, do they?

[Argos ain't have a care in the world about that, happily accepting attention like the oversized puppy he is.]

You can also find Vulpix there, they're another good option for Fire types, though Growlithe end up stronger.