Bellossoms can not be caught in the wild. They are the third evolution of Oddish. You can catch Oddish in the wild in a big number of Pokemon games, but they are not available in Leaf Green. When Oddish becomes a Gloom, feed it a Sun Stone and he/she will become a Bellossom.
If you decide to give your Gloom a Leaf Stone instead, it will evolve into a really ugly Pokemon called Vileplume. However, as ugly as it is, it is not uglier than Gloom itself.
Please evolve your Glooms to stop the madness ;)