Where could I get fire sprites, such as the one used in Life of A Merchant?

There's one at wadfather.... you'll need to make the sprite yourself, tho. I'd suggest making the sprite as a 3DO, instead of a .spr... That way, you can give it translucency.

...yeah, and it also won't rotate off it's source.

For added effect, consider having three of them close together, of different sizes, right in front of each other with the first two being translucent and the last being opaque, and offset their frames by a couple of cells each. Just a thought...