You will have to search everywhere to find all the Pokémon in Pokémon Brilliant Diamond and Brilliant Pearl. You may need to find them in specific locations to catch them all, or you may need to contact other trainers to see if they are willing to trade options exclusive to their Pokémon version. Here's where you need to go to find and catch Bonsly in Pokémon Brilliant Diamond and Brilliant Pearl.
It is important to note that Bonsly is a Pokémon version exclusive to owners of Pokémon Shining Pearl. If you own Pokémon Brilliant Diamond, you won't find this Pokémon in the wild. Instead, you'll need to find a way to contact another Pokémon Shining Pearl player to see if he's willing to trade with you. For Pokémon Shining Pearl owners, you can find Bonsly somewhere on routes 209 and 210 at any time of day.
Unlike Mr. Mime, who can be found in the Grand Underground in both versions, Bonsly's evolved form, Sudowoodo, cannot be found over there. You will have to rely on Bonsly's location to evolve it into Pokémon Shining Pearl. For those who play Pokémon Brilliant Diamond, you'll need to trade with another player to receive Sudowoodo and then have Ditto mate with it to hatch an egg with Bonsly or receive Bonsly directly.