II use only two platforms, you can try them: Bountyhive and Bountylord.com
https://bountyhive.io/ is a good platform to start.
It is easy to apply for campaigns there and there is a lot of campaigns to do.
And https://bountylord.com is a biggest platform. U can choose many bounty of there. There are a lot of bounty campaign, remember check project detail and ico reviews before joined campaign, if not just waste you time