Monster Life Plants.

The easiest way I've found is to get a C-Rank monster of any type and just combine it with every other C-rank plant you see until you get a plant. Might take a while, but it's definitely the easiest and cheapest way since combining 2 C-ranks costs only 60 waru

There doesn't seem to be any specific combinations (except for Special monsters). If you are fusing 2 different types of monsters, it's more likely to the be the type you are using to fuse. (for example, fuse a ghost with a golem, you are more likely to get a ghost. slightly less likely to get a golem, and even less likely to get a random type)

That is honestly where all my Waru has gone. I've released so many monsters that I can't keep count. I suppose I'll keep trying..