Best bet if you don't wanna buy those is run void missions, do survivals and defenses. If you're lucky a alert mission would have one as a reward. Honestly those mods are pretty easy to obtain so it shouldn't take you long. As a saryn prime user myself I went with power strength and range. Reason being, power strength affects the damage of spores and range allowed me to affect more people. Tho with this build I have low power efficiency so I run with a friend with a Trinity. Check YouTube for better/different builds.

Unfortunately I can't check the codex to tell you which enemy drops these mods, the best thing I can tell is: Wiki and youtube